Did one my self on an old gt frame for mrs daznal.I cut an old knackered frame up,fitted the old disc adapter to the brake caliper,put the back wheel with brake disk on on to the frame,clamped the caliper onto the disk(using the brake lever),this assured alignment then welded the bit off of the knackered frame to the seat stay of the gt.Easy
Go down to your local dumpit there are thousands of old frames there.