Here is my <$20 solution that heats 4-5 large brew buckets and only took about a half-hour to build.
So I made up a batch of sugar wash and then built this box around it. I pitched the yeast in the ferment about 8 hours ago. The temperature only dropped as low as 24 deg C after the pitch and without any added heat the ferment temp is already up to 26.5 deg C. I already had one brew band so I put it on one of the buckets incase things cool off too much - but I haven't had to turn it on yet.
I built it out of a 1" foam insulation sheet from the hardware store. A 4ft X 8ft sheet is anywhere from $8-$20 per sheet. I bought the pre-scored pink panther FORMULAR sheet because many of the precut lines are already at the right measurements. I already had some HVAC insulation tape laying around, but any kind of duct tape would work well. Using a 4x8 sheet is convenient because it makes an insulated box big enough to hold four 7.9 gallon buckets or five 6.5 gallon buckets. That's convenient because four 7.9 or five 6.5 gallon buckets holds enough wash to fill a 15.5 gallon keg boiler twice for two stripping runs. And the cool thing is the way this is made lets it fold flat so it can be tucked away against wall in the cellar or something.
Just cut the pieces and tape the four 24"x32" sides together along the INSIDE of the 24" edges to make a square that is 32" along the sides and is 24" high. Putting the tape seams on the INSIDE EDGES of the square is what lets it fold flat. Then take the two 16"x32" pieces and tape them together to make one 32"x32" piece for the lid. Attach the lid to the TOP INSIDE of one of the walls to make a hinge the same way you attached all of the wall edges. Then with the leftover foam piece, cut some 2" strips to fit inside the bottom of the lid piece and attach them so that they just barely slip inside of the walls when closed. This helps keep the box square when the lid is closed and also helps seal in the heat. Stick a thermometer in through the top and you're good to go!
