Background
Basically, the problem is that the sparge water tends to stratify  in such a HLT- the water at the bottom is cooler than that at the top.  Assuming one just draws sparge water from the bottom, the water entering the mash tun will be on the cool side when sparging starts and will rise in temperature as the level of sparge water in the HLT falls.   This is an more of a problem with such a HLT than it is with a the usual HLT/burner below since the the heating element is 1)  more concentrated and  2) is above the bottom of the HTL.  It's also more of a problem than with a similair electrical rig used for boiling wort since less heat energy is used which reduces the convectively driven circulation of water. .

Solutions
I usually (see 2 below) use a powered stirrer in the HLT.   It's the same one I use on the electric boiler but I swap the blade for a shorter one since the HLT is not as tall as the boiler.   Otherwise  (see 2 below),  I simply ignore the stratification.

1.  Stir.  For an electric HLT, the stirrer doesn't have to run all the time- it could even be a manual one (you and a spoon :-) if you've the patience.  Ignore the stratification while you are mashing and stir starting 5-10 minutes before you want to start sparging.  If done manually, I 'd venture to SWAG that stirring 2-3 times should to get the entire volume of water to the desired temperature.  Locate the temperature sensor towards the bottom of the HLT so it measures the lowest temp.  Defintitely insulate the HLT and keep a good lid on it- especially if you do long sparges.  FWIW, I never sparge (5 gal. batches) for more than 20 minutes and usually do a quasi batch sparge by initially dumping in a lot of sparge water then refilling as the level in the mash tun drops toward the top of the bed.

2.  Simply ignore the stratification.  I done sparges during lazy brewing sessions when I didn't want to fool around with installing the stirrer, or listen to it grind away or when I simply forgot to install the powered stirrer!  With the HLT heater element controller set for 168 degF and the sensor mounted about 2" above the bottom of the HLT, the water going into the mash is around 163 degF as I recall.  The temp. rises as the sparge progresses but doesn't rise above the oft quoted 168 degF limit which is said to avoid releasing starch.  This is because I typically (i.e. when I remember!) cut the power to the HLT heater when sparing starts.  The run-off from the mash also initials drops  in temperature. then rises gradually.  As above, this is with a batch type sparge- YMMV with a fly sparge.   I've never been into keeping detailed brewing logs and conducting "real" experiments/traingle taste testing/statistically analysis of results, but, the brews turned out pretty much as I expected and, most importantly, tasted fine to me.   Also, a ditto on 1. above  on HLT reguarding insulation and a lid for the HLT!

3.  I played around breifly with a "shroud" affair I cobbled together from aluminum flashing (sketch below) which I'd hoped would suck the cooler water from the bottom of the HLT, direct it over the element and increase mixing as the crudely illustrated in the sketech below .  It helped a bit not enough to warrant playing around/tweaking the design of a shroud.  Basic problems are that the heater doesn't run enough to ensure mixing and the channeled convective flow isn't strong enough.

4.  If you use a RIMS with an automatic recirc. temperature control method similar to mine, maintain the HLT a bit below the desired sparge temperature and route the sparge water from the HLT thru the heat chamber to get the exact temp. going into the mash automatically.   Locate the sensor in the HLT towards its top so it measures the highest temp. and let the RIMS heater/controller tweak the temp. upwards as needed.   I've not done this yet since the computer-on-a-chip used for the controller's brain is a bit low on memory.   Maybe "someday" when I get around to using a PC instead...

5.  Same as 1 above but play around with the location of the temperature sensor so that it measures the average temp. of the thermally stratified water and give the HLT single stir right before sparging.  Experimenting to determing the "best" location for the sensor seems a PITA to me; YMMV...  Big ditto on the need for insulation and a lid from 1 above!

Tips
1.  Mounting the heating element as low as possible will help minimize stratification.  This also allows for better and easier stirring if you go that route.

2.  Dropping some food coloring, idophor or (my favorite and most handy) cigarette ashes into the HTL allows seeing the convectively driven flow within the HLT.  This helps immensely in designing and redesigning or otherwise playing around with a shroud.  All the stirrers I've played with

3.  A variable speed drill seems a good, readily available way to drive a powered stirrer.   When not brewing, it could even be used for drilling holes!  :-)