I have a fairly new Co-Ax and I did buy several Forster and Sinclair lock rings for my dies, but I still had some Lee dies to use but ran out of Forster rings. None of my reloading, or press use would put too much pressure on the aluminum rings so I removed the O-ring and drilled and tapped the ring for a set screw (nylon tipped). Yes, the rings are narrower than the slot and they will move upward a bit. But,
they will move the same distance every time, so there is no variation. I have been using some of my "modified" rings on my Lee dies for several months and several hundred (thousands?) of rounds with no variations, no trouble. I keep fergitin' to buy more Forster rings...

FWIW; the Lee lock rings work on a "pressure-jam" principle. The nut is finger tightened and the O-ring applies upward force, "jamming" the male/female threads together with enough friction to hold the nut/ring in place. Same principle as a spring washer between the nut and fixture/press surface.