Its nice to see some good thinking going on here, but I am a little worried about the concept of any form of spinner in an arena with a net roof or a single gap in the screening.
A Bosch 750 can get a disc with some pretty serious power in it, and limiting the diamtre isnt nessasarilly going to effect too much. Compare it to the old scorpion Jnr, 1 astroflight (similar/less power to a bosch 750), a tiny diametre disc and thats pretty powerfull! There's no way I would run that in an arena with a net roof after the testing we conducted during our arena build.

It only requires a single bolt to be hit by a disc to have a serious projectile, and after the few close calls in past events I dont think safety can be comprimised by running any form of spinner in a net roof (unless it gets to the point they will hardly damage wood).

I will try and think up some ideas that could make it work....