Do the cork wads hold tight under recoil?
They do as long as they're a little over bore size. They're a bit forgiving in my choked barrels (spring back to shape) unlike overshot cards. Not sure what punch you'd need for your 12 guage. I use a 20mm arch punch for my 10 gauge. I rechecked the cork tiles I punch from, and they're more like 3/16" thick, but feather light, and made up of compressed cork particles about the size of course coffee grinds. Before I experimented with the cork, I just used a 1/8" felt wad over the shot. That held up under recoil too, but is more expensive and doesn't break up like the cork.