Heating a small tub w/o heat tape?

Hello folks!
I did a search and couldn't find anything, so please forgive me if this has been asked before.

One of my new snakes isn't eating for me (has missed 4 feedings now, about 25 days since he last ate). I contacted his breeder and one thing they recommended was putting him in a small shoebox-sized tub, in a dark area.

He's currently in a 10gl (with UTH and heat lamp) and I am unsure about how to heat a shoebox-sized tub. I don't have any heat tape, and my smallest UTH is already in use under his 10gl.

Would it be possible to use a larger UTH (with thermostat) and simply lay the sterilite tub on top of part of it, or would I need to use a smaller one to attach to the tub bottom?

I would greatly appreciate any suggestions as I am totally at a loss where heating and tubs are concerned.

Thank you for your time and help!
Artgecko

Will the shoe box size tub fit inside his current enclosure? I think an opaque tub would darken his environment a fair bit and then you only need to run one more temp/humidity probe or may even be able to re-locate the ones you are already using.

I had not considered putting the shoebox inside the tank, I assume the UTH temp will just need to be increased to go through the extra layer of plastic.

Will covering part of the glass tank with a towel decrease the light enough?

Thank you so much for the reply!
Artgecko