Click here & check this out.

That's what I have done with my 3 gal. bins.

I have started large flower pots as worm bins this spring and am going to wrap them in burlap, put a glass jar filled w. water, with an immersible aquarium heater (adjustable), seal the top real tight. Wrap the jar with damp old t-shirt (to prevent the surrounding VC from drying out). A folded burlap bag on top of everything and as cover I use those large pot trays. I have used seedling mats attached to a thermostat before. However, with multiple bins this got way too expensive. Just bought 2 more of this: Fluval Marina Submersible Heater for Aquarium (50 W) from amazon for $13/ea. Don't need to worry about "outboard" thermostat.

Just some alternate ideas.