I use a pair of Thule Sidearm roof racks to hold the bikes in place on the PVC pipe "bed". Almost any type of roof rack will work, even for bikes with fenders.
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In my case, the ramp attaches to a piece of 4" PVC pipe which is part of the rack. I used a bolt on the ramp which fits into a hole in the rack. I cut a small piece of pipe which is used as a "collar" to make it easy to line up the bolt & hole. The ramp lines up with the tray on the Thule rack and the bikes roll easily into place.
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PVC pipe is very useful for a great variety of projects. It can be cut, shaped, glued & bolted into almost any imaginable configuration.