Personally, I smash the buckets or carboys together and wedge the thermister between the two. No tape is necessary.
Set the temperature for somewhere in the range that both yeasts can perform well, usually towards the bottom of the warmer yeast's range. For example, I have a batch with 1010 (58-74F) and a batch with 1056 (60-72F) going right now. I have my A419 set to cut the electric heater at 64F and off at 66F. Because it's a such a small space using a personal heater with blower fan, it usually overshoots by a degree or two before the thermister can register it. It cycles about four times per hour in a ~50F laundry room for less than a minute each. Short cycles aren't a problem with this type of heater. The beer comes out tasting great anyway.
Feel free to insulate and all that jazz, but it's really not necessary, and you probably won't get any noticeable benefit from the reduction of temperature swings unless you use a carboy blanket type heater as well. Remember that it takes quite a while for 5 gallons of liquid to catch up to ambient air temperature going either direction. In my case, my two 5-gallon batches have more thermal mass than all of the air in the freezer, and probably the freezer walls themselves.
I should note that if you're making ales and the chamber is in a conditioned space, the benefit is even less.