It's easily the longest application of any rubber ring. I managed to find a guy in Europe that had some 6" (or maybe it was 5-1/2"?), but they are starting to delaminate. The Pinball Resource only goes up to 5", which would be a LONG stretch (I like to go a little weak on the stretching).
It looks like this guy @ IPDB has added a pair of under-the-plastic posts and put in a pair of rubbers as triangles, and a single one for the existing center post.