Heat safe plastics for reptile tank?

My new snake came in last week and holy mama he was a lot bigger than I anticipated. I had to scramble to put an enclosure together but luckily had a heavy duty plastic foot locker the right size that I was able to repurpose. Size-wise it's working great but because the bottom and sides have some corrogation it isn't transfering enough heat from the heat mats, and the lexan plastic I put over the windows I cut isn't safe for a che so I'm really struggling to bring it up to temperature.

My plan is to cut out either a section or the entire bottom (keeping the feet so it stays raised) and replace it with something flat that will transfer heat better. But I'm not sure what material would be best to use. Please lend me your DIY wisdom! And if anyone has any suggestions about places that might be able to ship something to me since I can't leave the house that would be an extra appreciated bonus!

How large of an enclosure do you need? It might be faster and cheaper to get a tub with a flat bottom shipped than to get everything you need to replace the floor in the foot locker shipped. Plus how are you going to seal the floor, caulk? And where would the snake stay while the caulk cures?

You said you used Lexan for the windows. Lexan is polycarbonate which blocks a lot of IR radiation depending on thickness and type, so it wouldn't work well with a CHE anyway. If you can replace those with thin plexiglass a CHE or DHP would work as thin plexiglass transmits most IR.

3 x 2 would be ideal, the current trunk is a bit under that. It seems like all the foot locker type tubs have uneven bottoms for extra structural support.
I did a little more brainstorming and realized that I could use the bottom of a smaller tub, since I only need to have the heat mats on one side.
I'll use silicone to seal the floor, it's relatively quick drying and he's a real chill guy so he can hang out with me while it cures.

Nearly all plexiglass has too low a melting point to use with any che/heat lamp, they have a much much higher temperature than under tank matts since they use indirect heating.