I have seen a lot of universal sub-bases so my guess is that someone sells just what your looking for.
While you're checking around look at this post as it has references to a "how-to" on making a sub-base and a reference to where to buy "router stuff online". I could go get the post and list then here but you might enjoy the trip getting there.
http://www.routerforums.com/general-routing/460-centering-base-plate.html
Ed