Wow. Sounds like a challenge ... nothing store bought, hmm?

First off, I think I would use a larger can, like a 2lb coffee can. Cut it down to about 2 or 3" . The string should wind off it better.
I think you could attach it to the riser below your hand with a leather strap or two.

Bowfishing is a lot of fun. That' s what sold me on tradbows. So much more fun to shoot than compounds.

Good luck. Let us know how it turns out.