Geometry again. How many inches does the wheel of the bike need to be raised to avoid the bike hitting the hump? Note: not how many inches does the door need to be raised at the bottom, which will be many inches more. This is the issue and I imagine when I get to try mine this week that it would be only an inch or two for the lowest bike, so that is just short plank under the wheel at that point. It could be that the front wheel may need to ride up on a small lank inside the trailer at this point also with very low bikes.
This would mean no need for elaborate extensions etc. and we all carry short planks for under our stabilizers and jacks. Give it a try with a mate to help and see what the possibilities are. You could always mark the point at which they are to be placed with a small piece of tape if you wish.
I hope it may be an easier possibility.