I like the Happy Trails fairly cheap, a guy sells some similar ones
for $299???, you have to supply the mounts. HT mounting has issues with turning and dropping a bag, this can be cured though. I have a friend who lost one. The upper mounts can pivot loosing grip. I've tested mine on the highway sliding on pavement at 50-55 mph till stopped by a guardrail. The bag was removed, but was able to beat it back in shape to reattach.
You can keep the mount from rotating by adding rollpins to the top mounts and drilling little locating holes in the bag.