Mike..something else you may consider. Mudflaps from semi trucks.

They are tough as hell and if you hang a pair of them from the top of
the shell, they may well stop the bullets from a .22. Ive seen them
used to stop .38 Special bullets in home made ranges. They have to
hang and flap freely though. Ive also seen some thick canvas type
conveyor belts being used for this as well.
The bullet hits, the energy is used to swing the flap and the bullet
simply falls straight down, oddly enough. YMMV of course

Gunner

I tried a similar thing once and shot at tyres. Nearly killed myself. The
dam things just came back at me.
If you want to go the floppy rubber route, get hold of some old conveyor
belting. It comes in various grades and is free. Ask at any
factory/quarry/mine that uses it for some old stuff.

John