I have an anery motley corn that is about the same size, and yes, the 10gallon does seem a bit big and I have considered putting him in a shoebox sized sterilite container. He's eating, so maybe it's not too big for him.
Yes, it's fine to use flexwatt under glass, but be sure to have the flexwatt on a thermostat and prop the tank up about 1/2" or so for air flow. They are burrowing little feinds and you wouldn't want him to get burned.
I have a human heating pad under my corn's tank, set on low, with the tank propped up by little 1x2's. The temps stay a very steady 92 on the warm side and 78 on the cool with the humidity around 45-50%. I had to cover half of the screen with saran wrap.
Hope that helped a bit![]()