I have an Artesian toilet from 1988. While changing the toilet-to-tank flange/gasket, my toddler ran off with the rubber channel pad. I am desperate to find a replacement. It was about a 5/8 to 3/4" square with a v bevel in it and maybe 4" long. I never actually measured it, never expected it to need replacing. Without it the tank would rest directly on a porcelain ridge (not good) and the tank rocks (even worse). I tried cutting a replacement from some white cell shipping foam. It rocks and I am afraid to keep tightening... don't want to crack the toilet tank. So, I'm thinking the product was too squishy and my measurements were probably off. Please assist me in finding a substitute product, what kind of rubber(generic name so I can find some and not violate this forum's policy), and if anybody knows the actual measurements maybe I can cut a new own from some appropriate material. Thanks.

You can use some rubber gasket material, available at any auto parts store, plumbing supply store, or home improvement warehouse.