1/4" cork is about all the hardware stored here have - might even have it at Home Depot. The Home Building Supply stores and Home Hardware stores have it. I suspect they are all related.

I'd think that with the Card with a small amount of shot-buffer to cup the ball above the wad, would give a very good seal. The cork might or might not be needed. The 'buffer' would keep the wad from pushing up alongside the ball which would make it lose it's sealing qualities.

A ring of lube between the case and the top curve of the ball would serve, I'd imagine.

The push-through 'sizer' would be an easy remedy for the .750" ball.