I've never done so either and almost want to use the small feeding tank that came with my guy as a test run and see what kinda happens.
I've been given the advice that because boa's sometimes burrow it's not a good if they burrow down into the drainage layer. Sealing could make it more secure but maybe they could still find a way?
My little guy was burrowing at first but hasn't since I got the proper humidity and heat going for him. Was planning the same thing for my little dude! Was going to slowly build an bio enclosure from the start. Either way I'm building him a sick enclosure.