i built a bioactive 12x12x18 for my Phants but they're tiny little geckos like the size of your thumb so they don't really trample plants like what i imagine a BP would. i was meaning to get around to putting together a DIY thread for it and there should be ideas that u can apply to a bioactive snake enclosure. anyways a few quick things i've noticed, observed or learned...
1. u will not be able to provide belly heat. heat will need to come from above - either a bulb, CHE, RHP, etc. i guess if u want the more natural setup, this is more natural as the sun radiates its (light) and heat from above.
2. u will need drainage. so if u have a bioactive that means it will have plants and plants need watering. a way to drain excess water is a must. it's important for the plants and to maintain humidity and control mold/mildew, etc.
3. glass enclosure is preferred but i think u can get away w/ a nice sealed transparent plastic/PVC/plexiglass, etc. like an aquarium, it needs to be able to retain water at the base. the glass also helps to measure water levels.
4. bugs. be ready for bugs. even the helpful springtails and isopods still make my skin crawl but they are necessary for bioactive enclosures.
5. my Phants leave teeny tiny poops and urates that the bugs can take care of. (actually i prefer to remove the urates i find) but there is still some maintenance for larger animals and the waste they leave behind.
6. don't go overboard w/ plants IMO. i did and i think it made it makes it harder to find crickets. i image for a BP, u don't want them destroying your investment. just enough clutter for security and decor.
good luck and i hope to see a thread on your bioactive build.