Hey RG sorry, my bad, just re-read your initial post and I see your problem and totally empathise.
I watched the vid MJMP posted and it looked good but I'm thinking there may not be enough thickness to do the thread plug (MJMP, do they make them for such a "thin" hole ?) if they do then this is the best option IMO
Or you could go with an alan bolt next size up and re tap the thread for it, but as you said then you'd have to expand the hole in the clamp block (not an easy drill, best use a drill press with good clamps, or a rat tail file), however, this would work (and you might get away with not re-tapping the thread).
Or you could fill the mounting screw holes and fit a new locknut (from either the top or from behind, but as the original holes are so close they would need to be filled (plugged)) and also you haven't been able to find one "exactly" the right width, so a bit of work and still not optimal.