Submersible heaters for aquariums don't reach the temperature required for colour processing (38 C) because fishes would die. If you develop colour that means you'll have to "modify" your heater (break it so that the knob turns freely beyond the maximum temperature).

I think for your case the best is one of those food-warmers with a thermostat. I assume you only need one tray with really correct temperature. You will probably be able to find some in a second-hand store or used on the internet.

Something like this:

make sure there is a thermostat.

No water, no light (or easily maskable) and no great expense either.