I ordered a disk brake conversion kit (just the bracket). Unfortunately it placed the calipers at an undesirable angle and position. I designed and printed a collar/shim that would 1) take up space between the over-sized mounting collar. and 2) provide a straight and vertical position for the collar to grip rather than the angled position of the frame itself.
The end result is a brake caliper that is positioned correctly and was much sturdier than with rubber "sleeves". As an added plus the ABS plastic seems strong and durable enough to remain permanently and it also protects the finish on the frame from the steel collar that attached the bracket to the frame. Notice the B is for the bottom (more narrow tube than at top) and the Line is on top (to get the angle correct).
(I didn't realize and ended up printing mine with only 10% infill! ) So far it seems to be holding up well, however, when it comes time for you to print I recommend using a higher % (Perhaps 50-100%)
I enjoy making old things "new" again and the current standard in MTN bike brakes are disk.....so why not have some fun!?
This project should work for many other older (80-90's) steel frame bicycles as long as the size of the tubing in the rear triangle is of a similar diameter.