Step 1: Camera Choice
Step 2: Fitting mount to frame
Step 3: Fitting clamps to rear stays
Step 4: Pan and tilt.....
i had a bunch of ideas and eventually went with this one. in effect it gives pan and tilt. by rotating around and tightening against the horizontal rod (the one fixed to the bike) i can tilt the camera up and down then hold it in place, byb rotating around the shorter rod i can get tilt. I used a cute little aluminum block from some old optics kits i've got but you could make one of these easy. drill 1/2 in holes at 90 degrees to eachother in an aluminum block then drill and tap two fastening holes for bolts, one that enters into each hole.
this is the expensive version of these clamps but i'm sure you can see how to do a cheap version: