There is a guy on Trapshooters.com who makes bore gauges for 12's for $95 (or at least he used to). If you could put together a group order, he might be able to build something for 20's (maybe through 28?). I have one that is .640-.800 that I used for 12s and 10s and am very happy with it. His name is Kenny Helderlein. If anyone wants it, PM me for his contact info assuming it is still current.
I have one of his gauges. To be honest, I think they are a better quality gauge. As easy to use as the Skeets (I have a Skeets also). The fellow who makes them is great to deal with. I'd recommend them any time.
Good Luck!
Greg
