both a2z, hope and woodman did rear disk brake adapters, these usually needed fairly flat drop out areas and you'd need to see if they'd fit your bike. They usually included a torsion bar up to the cantilever mount, certainly the hope and woodman ones did. I used one of each on my bikes and they worked well in terms of adding disk brakes - hope brakes in my case. It has to be said tho, it did take longer to change a flat and was a bit of a faff, BUT the power of the disk brake we ace. I eventually got disk mounts added to the frames.