Have you considered a tray-style, receiver-hitch mount rack?
It's ridiculously easy to load and un-load the bikes and simple to remove the rack from your rig. Sell your new rack and put that with the dough you'd spend on a make-shift top bar and get a tray-style. Prices and quality range from cheap stamped steel to units that are made of Unobtainium and put themselves in the garage.
Just a thought. I know those tray-style racks don't work for every application, but they're super convenient.
A random example:
