If I understand this right, (after re-reading, because I had an elaborate system typed out, went back and read your post, and decided it was all wrong!), what if you take 4 fender washers with an inside hole diameter that is smaller than your cross dowel diameter, drill 2 small holes on the outside edges, and screw it, centered, over the hole, so that a flat blade screw driver will fit through the washer to assist with alignment? So if you used a 3/8" cross dowel, use 1/4" fender washers. Should be able to get a decent sized screw driver into the slot for adjustment.
If this doesn't work, just a flap of plastic, with 1 screw into the bench, that will rotate over the hole or out of the way for adjustment. Would keep the cross dowel from falling out of it's slot, but allow access for alignment when needed.
If both of these are off base, let me know and I'll retype my elaborate system! Jim.
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