I'm sorry, but this is one of the worst Nerf ideas I've ever heard. There's a reason mass production was invented. In this case, it's so you can pummel your opponent with a barrage of foam. While you're sitting in your bush, swearing your head off while you try in vain to reel in a nerf dart tangled in some sort of obstruction, somebody's gonna snead up behind you and nail you in the back of the head with a wildfire. Honestly; if you can't make stefans, refill dart packs aren't that expensive. Go buy some, and be an actual competitor in your next nerf war, instead of cannon fodder.
And if I'm thinking about the piece you're talking about, maybe you could take one off another busted wildfire, rf20, or PC...Basically anything with an air bladder attached in a similar way.