Hey there! Big builder of cages here and lover of bioactives! For spray foam, personally whenever I do bioactives I use GREAT STUFF brand big gap filler or regular gap filler, it’s only 3 bucks a can usually so definitely affordable too! As for applying, most my cages are too large or are set up in a way that they can’t be tipped onto their backs conveniently to spray foam them but for a 40 gallon I’d definitely recommend it, I’d also recommend spraying outdoors or at least in a garage or highly ventilated room (doors open, windows open, vents open, fans on so no air is stagnant) while the afore mentioned great stuff foam puts of little fumes, safety first! I’d also recommend keeping any animals away from it while applying and letting it expand and harden. If you’re looking to get a very built-out background I’d recommend putting on a base layer then adding any wood, hides, or styrofoam to use as a base for anything your snake can hide in or lay on, for example in my 6 footers I used plant pots on the background to have vertical space for plants too along with putting in a crab apple wood log for climbing with some other branches and large roots. The best part of great stuff is generally you can do all the foam in one go instead of layers! As for silicone, you can buy silicone but it gets pricey FAST, trust me, talking 300 bucks to cover two enclosure background for me. Although that was for a large amount of space I just don’t trust silicone either. I can’t speak on silicone because of this but I’m sure it’s a good option since it’s widely used, what I use instead though is a nice 20 dollar can of Valspar 2000 black which lasts a millennia’s worth of backgrounds for me, base coat then left to dry and finally another valspar coat which I fling dirt and moss into to give a 3D effect. In all honesty I can get this done in all 1-2 days so backgrounds are generally pretty easy!
As for substrate, I frequently use peat moss tourbe, although it’s not sustainable and I do not recommend it I bought a bunch and have been stuck using it up until now unfortunately. It does work great but since it’s a non-renewable resource and is an important resource where it comes from I try my best to avoid it, instead biodude is pretty good as others have recommended to me and local expos often carry homemade bioactive terrarium blends with starter plants you get along with it or you can also purchase, or might even come with isopods! For homemade mixes I personally enjoy doing a mix of top soil organic dirt (no perlite, fertilizer, or anything in it besides just dirt) dried sphagnum moss (Home Depot sells it in baggies usually, the more course and less processed it is the better) wood nuggets made from any animal safe wood (non sappy and generally non coniferous) or orchid bark which is commonly used and is made from safe coniferous trees, although if it’s smelly to even a person, beware! Another thing to mix in or simply lay on top is dried dead leaves, walnut and magnolia leaves are my favorite, but most any will work if properly treated if you found them instead of buying (letting sit out to dry and generally picking clean looking leaves, then popping into the oven at 450F for 10-15 minutes)
Onto water features! Fountains are a fun addition but I’ve found that pythons have to tendency to wreck them, I’ve tried soooo many times to little success, so I just stick to good ol doggy bowls from the dollar store although faux rock water dishes would obviously give a more realistic look, I tend to put mine smack dab in the middle or more towards the cool end, this is so any snakes that want to bowl wrap can do so comfortably and cooly but so that the water will still evaporate and isn’t interfering with where I like to put hides and plants on either far end, that’s my note on water, I’d also recommend 70-80% humidity for a ball python bioactive and slightly warmer temps than usual to accommodate for tropical plants, as for watering follow the specific directions fire each plant but do not let the ground get too soggy while watering plants to prevent scale rot (basically wet enough you could wring out the soil and some water would squish out if you grabbed it and we’ll, squished it but not so much you so much as poke it and water gushed out)
Isopods! Some of my favorite stuffs since having a bioactive gives you the excuse to get more pets, aka domestic pill bugs 
as for isopods, there isn’t really a clear answer since they’re all great, it simply has to do with color preferences, morphmarket has a good amount isopod sellers to check out but also some Google searches should suffice for finding tons of variety to chose from, popular ones include dairy cow and rubber ducky meaning they’re sold commonly, but there’s other rarer much more expensive variants like clown and Thai spiny which are definitely worth the price if you’re into isopods, so it’s totally up to you, go for whatever isopod coloration strikes your fancy just like picking out a ball python morph to buy or breed! I will say though along with isopods you should look into getting springtails which are basically a secondary cleanup crew to isopods which aren’t as essential but I’d say definitely key to having a well set up full enclosure, another recommendation but is not necessary is predatory mites, there’s tons of species so you’d have to research as to what suits you best but they can help with anything that may be plaguing your plants or affecting your snakes (like snake mites or spider mites!) although expect if you get certain species to go after springtails or even isopods every once in awhile, and generally they’re only used when one does have an issue with problematic unwanted mites
Unfortunately I do not have any recent pictures of my 6 footer bioactive nor my other bioactive ball python enclosure because as of recent I have been dealing with a major mite problem that plighted my entire collection so I had to tear a bunch of stuff up since predatory mites aren’t as available to me. I also don’t have pics of the bioactives I’m working on since they’re still heavily in progress and the one that is more completed’s occupant has gone missing for the moment so all work on that enclosure is halted until I find the escapee. Hope this helps
