I cobbled together a "carrier" for a pair of folding racks this summer. This was done in a hurry before a trip.
In fact, when I really want to get something done, I figure the best way to do it is to schedule something else so that there's suddenly a little urgency involved.
I started out with just a notion of what I wanted to accomplish, but no real plan. I basically looked around the garage and basement to see what materials I had on hand that might work.
In the end, I used a 2' x 2' piece of 1/4" plywood, some cedar lathe, a piano hinge, a piece of heavy broom handle, some nylon line, woodworkers glue and a few screws, bolts and washers.
This way the bikes are stored out of the rain, behind tinted windows
away from covetous eyes, in a locked vehicle with an alarm. They are available if we drive somewhere without the trailer, are readily accessible for loading and unloading, and they don't block visibility through the rear windows
when we are driving.
The carrier held the bikes steady crossing over the Cascade mountain passes, so I'd call it a success. Naturally, I'd like to tweak it a bit to make it a bit more elegant, so I guess I'd better schedule something else to do now...