Don, when I was driving stock cars a few years back, we were running two tracks which necessitated changing the chunk in our Ford 9-inch sometimes twice a week. We never used a gasket; we just used high-temp RTV silicone, the copper-colored stuff. Never had a leak and never had a problem...

As an alternative, one time back when I was kid, I watched my dad make a gasket for a differential from a brown paper bag. He used the chunk that had been removed and "cut" the new gasket by holding the brown paper in place and tapping the edges with a ball-peen hammer. I've never forgotten that trick... If you can't find a gasket locally and Dan doesn't want to just use RTV, you can make a perfect fitting gasket using the above method. If Dan isn't happy with brown paper, he can get some sheet gasket material at the parts store. (Gotta be pretty thin for the diff, though...)