I had a traumatic evening on Sat, fitting a rack to my orange clockwork as the disk brakes made this very difficult (avid BB7). I eventually got it to work by trimming a bit of the rack with a hack saw and spacing out one side, but this means the rack is not central and more over to the left than right.

Any advice on a better solution?

http://www.evanscycles.com/products/tortec/p-clips-ec006759

P clips are your friend , I have used them to fit racks to bikes with no mounts , you'll just need to mount the lower rack eyes a little bit higher.