Several years ago I had made a setup similar to the one your are considering.... except I mounted it to the front of the motorhome. It was an old 454 gasser and I never had problems with weight or overheating.... but I digress. On my setup, I What I did to facilitate single handed loading was to get the bike up there in position and tied down. Then I fabricated a couple of upright angle iron pieces that were welded to the base channel and were notched out so that the front footpeg mounts fit into the notches.
Now, when I slowly ride the bike up the ramp and onto the channel, it goes forward until it slides into the notches in the uprights. That holds the bike in place while you attach the tiedowns... worked like a champ for many years.