The best shellcatcher bands were the ones you can get at the Post Office. Wider, thicker, and about 3" in diameter. They fit tight on an 1100 or SX-1 and will last for many rounds before needing replacement.
Any trap shooting mail carriers out there? If so, you know which bands I'm talking about.
If you are ever in need of a shell catcher and have nothing, and shoot an 1100, take one of the metal rings out of your gun. The bolt will not open fully, and all you do is manually open the gun to extract the shell after firing.
I've used the rubber band shell catcher method for years on SX-1's and 1100's with great success. Just remember to remove the band when your finished shooting as leaving it in place for an extended period will effect the bluing in a negative way. Don't ask me how I know!