Back in the 1980s I built up a Dawes Super Galaxy frame using parts that I had. The frame called for a long reach brake caliper in the rear, but I didn't have one. So I made an aluminum bracket to mount a non long reach caliper. I believe it was made from 6061 aluminum . I'll see if I still have the braket and if so take a photo of it
Hey
Pau keep in mind the date of some of these threads. This one is not that old but I don't think the OP has been back to look at their threads and trying to DIY brake attachments is never a great idea to begin with as someone could get hurt easily doing it wrong.