For adult burms and retics, the SMALLEST I would go is 8X4X2. This size is okay for anything up to 15 feet. Anything larger and I woud definitely go bigger. The idea of a 20 foot snake in something 6 feet long is akin to animal cruelty in my opinion. Especiallly retics, who are super active by nature. Everyone who owns one knows how exploratory they are when handling; why keep them in such a tiny enclosure? I suppose it depends on your outlook. Are you keeping these animals to enjoy them or to simply make a buck?

I use melamine, and if you seal the edges with aquarium silicone, and then duct tape over that, you will have an enclosure that will last a good number of years. Be sure to include a brace at the midpoint. I cut a U shape and it runs, upside down, from the front to the back.

Again, these are my opinions and that's what you asked for.

Chris