Mark_h4dng
You could buy a 28.6 clamp, and get it Machined out to the size you need. 29.4 is only another 0.8 Dia bigger and wouldn't weaken it very much.
Or get a 30mm and use a bit of coke can, or be really classy and use a jubilee clip (although that may limit the quick release potential a bit). Suppose we should remember it is a Sunn bike and like all things French it has to be a little different!