The mounts I used have the ability to mount on flat surface, or round tubes although I don't believe they will go out to 1 1/2 w/o mods). The trouble with mounting them overhead, is that you have to hold the bow up while using the velcro wrap to hold them in place.
I have been using them while up at White Pass for last week, really like how convenient they are. There is just enough room to close the back door, and still have clearance at the roof. And the bow mounted Quiver also helps keep the bow from getting smacked while loading the back area.
It was still to warm up there for good archery hunting, but had a good time. Headed for Montana towards end of week!