<< Agrees with using a PVC type material in building your cage.

For size, there are quite a few options. I'd recommend going with 36 wide cause the 30 as a 20L tank is won't be enough unless you are going to make the enclosure deep. Depth, I'd go 15-24inches deep. For height, they don't climb much when they are older/bigger so 15-18inches is fine.

This is my opinion and what I would go with/recommend. I recently purchased a load of 36x24x18 PVC cages for a variety of snakes, 4 of which are BPs.