I was recently asked for my plans for a disc brake adapter I made for a neighbor-kid's GF a few years ago. I couldn't find my drawings; but I scratched around in my garage and found my first prototype and measured it up for the requester. After I got done and PM'd him, I thought maybe more people could benefit from my little bit of ghetto/shade-tree machine work. This adapts 51mm IS disc brakes to the old 22mm vertical-holed chainstay mounts on Treks and GFs.
I know, the store-bought ones don't cost that much; but if you're handy and happen to live near a city with recycling yards that will sell you aluminum scrap by the pound, you can make one yourself for a little bit of nothing.
I made this from a piece of 1½x1½x¼" 6061 angle. The 22mm holes need to be pretty close to the inside face (11mm), so with that thick of walls, you see I needed to relieve some vertical 'troughs' to accept the bolts/washers/aircraft lock nuts. I did these with a straight-shouldered bur bit for a Dremel. You could freehand it with a Dremel, of course, or chuck it in a drill press and 'precision bore' them if you choose.