Build one. I wouldn't take a chance on any kind of critter nests with a ported sub, I'd go sealed. Since you said "passive" that makes it alot easier. You aren't trying to bury it in the ground, right? Find a driver that is moisture proof and make a sealed, downfiring unit. For the enclosure, you could rely on paints/coatings to protect from the weather or you could use ground-contact rated treated wood. Do not use MDF, particle board or OSB...it won't hold up.
Or you could get creative with it and use sonotube to make a concrete enclosure! Would be pretty heavy though lol.
If you end up doing this, please report back!