I disassembled a Topeak seatpost mounted rack. I didn't have to destroy the rack either, just removed two screws and slid the rack off the post. It makes for a great mount since it already has a seatpost clamp on it, is angled slightly, and is not a tubular so the mount doesn't slip. The video is turned out really good. Unfortunately I uploaded it to facebook so I cannot share it right now. When I get home I will upload to youtube and repost. I just thought I would share in case you have one of these racks laying around.
I hope you put an aluminum pin in the mount so that it can peal away if you catch it on a tree or fall on that size. If the mount was too ridged it could put a lot of stress into the frame in a direction its not really built to handle it with that long lever arm.