jacksbike

This type of rack is made to fit tight against the seat stays of the bike, below the seat post clamp. I am not aware of a way to make this fit against the seat post clamp without you making your own steel U-shaped device. If you have one of the road type bike racks that have individuals tangs coming out , made to fit into brazed on attachment holes on the bike's seat stays, you could rig up a couple of extra steel pieces to fit against the seat post clamp.
To explain the hardware that you have to use with this type of rack: The 2 long bolts go through the flat end of the rack itself. Next you put the bolts through the flat piece with the 2 holes drilled through them. Next, on the inside part of the bike's seat stays, place the single piece with the dip in the middle, with the dip pointing out to the rack. Lastly, the T-piece goes on with the bottom of the letter T fitting into a bolt that goes through the bike's short welded piece that connects the left and right bike seat stays. The long 2 bolts now go through the flat steel plate, than the bike frame, then the steel piece with the dip in the middle, then the upper part of the T piece. Fasten with washers and nuts. Sounds a lot more complicated than it is.