Not really sure but shooting 600 yards yesterday I ran out and was hitting the 8 or 9 ring. So just looking for something to get a little more out of it till I upgrade scopes later.
What make and model of scope? Pictures would probably work.
Edit: I paused and did the quick math. If you were out of elevation at 600 I share your concern of having too much elevation to keep a 100 yard zero if you put on a 20 MOA base. You probably only had 14 to 16 MOA on the scope and if you were out of travel, the 20MOA base will get you to 600 easy, but you wont be able to come down enought (probably) to keep the 100 yard zero.
So, to answer your original question, I am afraid I do not know of anyone that makes a 10MOA base.
I have one of these on one of my rifles and it works fine.