What do you think about using fish tank heaters?Make the larger one out of 5 gallon bucket. Only put 2 gallons of water in. Hang a heat lamp so it is just even with the top of the bucket and plug it into a thermocube. Mine never hit it hard enough to make a splash that would come close to reaching the lamp. As long as the lamp is secure enough to not fall into the water, lol
If you want more security, put in a GFCI in place of the regular plug. The most that will happen is it will trip the GFCI if it ever fell in or got wet.
I can find those for about $7 apiece
I have to find something to make about 20 of them.