I have used a car top bag like the one pictured on my small Pontiac Vibe with much success. Make all luggage soft type and put all of that in the bag. Models and heavy stuff go in the car. I have use it in conjunction with the same cargo carrier from Harbor Freight that is shown also. I cut a piece of plywood to match the outline of the bottom of the roof top bag. I did MOT assemble the cargo carrier and left it flat. I attach the plywood to the cargo carrier and then the bar to that assembly with tie down straps. This all goes in the receiver on my Chrysler minvan. Same plan here, all soft luggage goes in the bag, and models and flight boxes go in the van. I think my bag is a Thule and I got it from Sears, which is hard to do these days, but I'm sure they are available some where else. The one depicted here in this thread is very similar. They are quite water proof if used correctly.
. Type at you later,
Dan McEntee