Nice. I lack welding skills and a place to set up some place to learn. I can get stuff 3D printed in industrial plastic though.
What I have in mind is offset arms. By varying the distance between them and their angle I should be able to mount most anything as long as it's not too heavy. If 4 arms are placed at different clock positions, i.e. pointing towards or away from a center, then the distance between the ends of the arms would change if rotated. If the distance is fixed because they're bolted to something then they can't rotate.
Point them all in the same direction then you have Maxtrax twirling on the roof rack like the tassels on a stripper's pasties.