I did something like this once for a garage beer fridge using custom cut plywood. I realize it's not the most sanitary idea but hear me out. I got good quality board dense board. Then I painted it with 1 part epoxy to make it as moisture resistant as possible. there was really only cans of beer & pops in there, so I wasn't concerned much. I never had any mold or mildew because I just made sure it was clean and dry. It's a cheap option and you can cut it exact if you have some wood working skills. This is reddit so I would expect a million down votes about how dangerous and irresponsible this is but blah blah blah.