I just did this last month, like Mr. Fromm says the ebay search key is "graflex focusing panel" and the statistics you need are "2x3 Graflok back."

In fact, this http://www.ebay.com/itm/GRAFLOK-FOCU...item5401f8ea48 is kind of what you're looking for. If you'd like to compare measurements from one on a live camera, PM and I can grab a tape measure. It's on the back of this '62 that I own: