Magnet Mount Lights on Towed Car

I have never even thought about this issue however, you query has triggered what may be a useful idea. I have in the past purchased cupboard and drawer liner (plastic or rubber mesh) for use in our MH and also in a boat we had. The stuff really impedes/stops the sliding of anything placed on it. I also use it on my granite counter tops to stop a cutting board from slipping. I have no idea what it is called however I think I bought it at an RV supply house. I would think that placing that stuff between the lamp and the car surface would protect the car surface and probably do some good as regards decreasing the probability of the lamp slipping. I would guess you could glue the material to the lamp base but removal and replace may be a bit of a problem. The material is quite porous and the glue used would likely get a real good bond. Also, the hardened glue may also damage the vehicle surface.
Just a thought!
Cheers;
Vic