I’ve seen your type of adapter for sale with the caveat that it only works on vertical dropouts
No johnners, that one is designed for horizontal dropouts (the chain tensioner is on the inside of the dropout). For some reason the internet archive site is down at the mo so I can’t search for my old posts.
I bought mine (Quad QAB, iirc) from Raleigh UK but it no longer seems to feature on their site. It was also available from these folks but again doesn’t seem to be on the new site. If you do manage to get one, it will only work with cup and cone hubs (the cut out in the main part of the adaptor sits over the axle cone nut, the other side is knurled and the tensioner sits next to that on the inside of the dropout). I was a bit worried that my axle would be too short as with everything in place it was barely protruding into the dropout but the tensioner kept everything in place. I did try fitting a thinner cone nut and spacer but this just threw things out of alignment and the caliper ended up rubbing on my spokes. With everything fitted it only takes up a few extra mm of space and I reckon with a longer nutted axle it would be perfect.
Oh and MussEd, the Drop Offs are 130mm (old ’04 model) – it was a bit of an experiment but I quite liked the ride. Back to 400mm Planet X rigids at the mo’ though.