This is a dangerous test of my memory, but as I recall, isn't the RMC aperture pretty big? I just can't remember the one I had well enough to do more than speculate. But if you wanted a smaller aperture and it's big enough could you tap it, or drill and tap it to accept some of the standard screw in apertures from Lyman or TC? Or if its' too small and you can weld, build up around the outside of the aperture to allow drilling and tapping? The size and thread on all those inserts appears to be standard. Idle spec. Acourse, if you could weld, there's also the possibility of filling it in and redrilling to a smaller hole.
