I decided to look around the house as there's a ton of plastic storage tubs lying around and found a nice (mostly) transparent one at 19"x13"x11" with straight walls for maximum floor space for the size. Perfect for a baby I assume? Just needs some air holes drilled in. Only other extra thing to add would be some clamps since I'm sure it could be opened easily from the inside as it's just a simple snap-on one. And there's plenty of bigger tubs to choose from as it grows or if what I found is too small.
Question is how does a UTH work with a plastic tub? Do you need to elevate the tub at all and/or other steps so it doesn't melt the plastic or is that a non-issue as long as it doesn't overheat?
And as for the holes themselves, how many is generally recommended? Like a couple rows on the lid and a couple more on the sides? I do know that you can cover or uncover holes as needed with maintaining humidity (which would mean I can more comfortably go with newspaper now that I think about it).