+1 on checking with your LBS. They may very well have some of the needed parts for making such things fit. When I bought my rear rack, they had a lovely little piece that fit around the seat post that the top portion of the rack could clamp onto (since my bike does not have any of those mount-points on the frame by default). If you do get extender pieces made, make sure that it is not made of flimsy scrap metal, but of something tough and durable.

If all else fails, your other option would be to actually bring your bike into your LBS and see about buying a rack that does fit (the one I bought is highly adjustable, so that I was able to extend the bottom "strut" (for lack of better term) length, and adjust the top stabilizer arms that clamp on near the seat post for a good fit. Granted, this was a more heavy duty rack and cost me about $50 as opposed to $15...