The 36X18 of a 40B can accommodate a lot of snake length, and still leaves head space for some climbing. I like the reptile ones with the slide out screen lid (but not sure how secure that is with more powerful animals). You can also get a very sturdy metal cover with metal clips for many of the basic aquarium measurements off Amazon.

I like tanks, but my North American native snakes tend to hang out in the open much of the time anyway, so they make for good display animals. The next enclosure may be a tub simply for space reasons. Planted tanks work well for my king and corn who need more humidity than we have here in NM. People do the planted set-ups differently, but IMHO you need a good drainage layer and a top screen to keep the substrate from getting soggy if you add plants.