CAVEATS
First, don't plan on storing brew in the mini-kegs for much longer than a month or so assuming you keep the bottles cold. YMMV with delicate brews. Long term storage will cause your brew to be noticably oxidized since the PET plastic the bottles are made of is not a good O2 barrier. Yes, the pressure in the bottle after filling is much greater than atomspheric pressure, but, as counter-intutive as it seems, oxygen will still pass through the plastic.
Second, the tire air valve stems I use are made of chrome plated brass. Neither is good for contact with beer. I remove the chrome on the bottom part of the valve and "rationalize-away" the exposed brass since it's only in contact with the brass if the bottle is tilted or shaken. A soak in a vinegar solution may help remove the surface lead from the brass. Also, you may have to shop around a bit to find this type of air stem and they cost around $2 each.
Third, I've only used plastic bottle caps. Aluminum bottle caps should work tho'- maybe even a bit better.
Lastly, the compressibility of the gasket material is pretty critical and you'll probably have to shop about to find the right stuff. Most of the usually available material I've tried is too hard. You might also have to modify the thing to get a good seal (details below).

CONSTRUCTION
Here's a drawing of what the keg top should look like:

First, make the brass plates. Locate the holes for the air valve stem and the 1/4" copper beer-out tube. They should be as close together as possible to enhance the seal. Allow enough room for turning the air valve's nut and for the air chuck you plan to use for pressurizing the finished keg. Drill the holes for the air valve and the copper tube in both plates. The holes in the bottom plate should fit the air valve stem and the copper tube tightly since they'll be soldered directly. The holes in the top plate need to be a bit larger to allow for easy assembly. I drill both at the same time by stacking and clamping the plates then drilling. I then wallow out the hole in the top plate a bit with a file.

After you've made the plates, shape the bottom one so that it fits inside the bottle. You want to make this plate as small as possible so that it seals well against the gasket. Note that the gasket extends a bit past the lower brass plate. Now, locate the holes in the bottle cap using the holes in the bottom plate as a template and drill the holes slightly oversized. Make the gasket. Again, use the bottom plate as a template. Using a fine-cut file or fine sandpaper, remove the chrome plating from the lower end of the air valve. Size the protruding lenght of copper tubing so that it and it's attached vinyl gas line doesn't interfer with the air chuck you plan to use. Using no-lead solder and as little of it as possible, solder the air stem (after removing the core) and the copper tube to the lower brass plate. Clean off the flux and assemble. Putting a bit of oil on the air valve threads and on the washer enables you to compress the gasket easier.

After assembly, do a hydro test on the assemblied keg. Don't skip the "hydro" part and use just air or co2- it's a safety thing. If something ruptures during the test with water filling the bottle, less energy will be imparted to targets in the blast zone since water is practically imcompressible. Alot of energy is stored in the gas contained in a gas-filled keg. Replace the air valve core, fill the bottle with water and secure the top to the bottle. Pressurize with air or co2 via an air chuck . I use 30 psig but once took one up to my water tap pressure of 55 psig without any problem. You'll note that the bottle expands a bit and there is a bit of air or co2 atop the water in the bottle. This violates my advice on hydro testing but it's only a little gas space (< 1/4 cup if your top is tight) and it's actually a good thing since air or co2 will escape through any leaks more readily than water would. Assess leakage by applying a soap solution to the bottle lid gizmo. Alternatively, let the bottle sit for awhile and assess leakage by noting the hardness of the bottle. Assumming the temperature doesn't change and you've done a good job, the bottle should remain hard. If not, first try tightening the air stem's nut a bit then try a softer gasket material. Removing a selective 1/32" or so layer of the lower brass plate also helps. I use a Dremel moto-tool and leave a 1/32" or so high by 1/16" wide "ridge" at the very outer edge of the lower plate. This enhances the seal since there's a smaller contact area between the brass plate and the gasket. Of the two tops I've made, one required this relieving. I suspose one could also make a good seal using food-grade silicone sealant by smearing the stuff between the lower plate and the cap's regular gasket, loosely tightening the nut then, after the silicone has cured, tightening the nut.

USING THE MINI_KEGS
I usually counter-pressure fill the bottles from a corny keg. Plans for my homebrewed counter pressure bottle filler are accessible by clicking here. A caution tho': if your experience with counter-pressure filling is limited to regular glass bottles, be prepared to use 3 times more force to hold the filler on the larger diameter 3L bottle tops! (I got careless with a ginger ale at 30 psig and got showered a bit). When I'm filling more kegs than I have keg tops, I cap the bottles with unmodified plastic caps then replace this cap with a keg top when I'm ready to tap it. Preserving the brew's carbonation during the filling operation isn't as critical as it is when filling bottles since the carbonation can be "topped-off" by applying co2 via the tire valve. For applying CO2, I like to use a clamp-on type air chuck. A regular type which you have to hold onto the stem would work but then you have one less hand free. The cheap ones which use a lever as a clamping device allow (IMHO) too much pressure to escape when they are removed. I perfer the type that uses a axially sliding sleeve to do the clamping. Both types of chucks (at least the ones I've tried- YMMV) have slow leaks when connected, hence, I usually disconnect them when not actively pressurizing or dispensing.

Flat brew or pop can also be force carbonated in 3L kegs but it's a bit of a drag. If force carbonating in the 3L kegs, a "Carbonator" (tm) type gizmo made from a tire air stem stuffed in hole in a PET bottle cap is a good idea. FWIW, I use the same, all metal tire air stems for these tops rather than the more common rubber types- the strong rubber smell is very difficult to remove from the later type air stems. I don't naturally carbonating in PET containers due to the potiential for oxidation of the brew.

For dispensing when the contents will be consumed in a day or less, I use air via a foot operated pump. I also put a small brass check valve in the air line since those in pumps sometimes leak. I also put a air gauge on a tee and insert it in the air line to help assess the dispensing pressure. For longer term dispensing, I use co2. When the bottle is full, co2 has to be used with every glass dispensed. As the keg is drained, you can get by with only boosting the pressure back up every second glass.

ENJOY!