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Fuck you, dildos, I made one!
Even though you fistful of assholes didn't give me a direct answer, you still answered my question.
Turned an army surplus waterproof duffle bag liner into an ass flap. The grommets at the top allow it to clip to a belt with ALICE clips. Took it out hunting today and it works perfectly. Total cost: $9. If you want to make your own, ended up being about 24"x15"