I have double radiators, and the gap between the panels is just the right size for the lower half of a coke can to nestle in. In any room in the house that I spend time in, there is a can on the radiator. Its held pretty firmly, and presents no spill hazard, and is barely noticible as near the ends of the rads the fins are more deeply recessed.
I first cut a circle round the can, then lots of 5mm cuts along the axis of the can to allow me to fold the sharp edges down. that makes a big difference to how likely they are to cause a cut..
I top them up with a watering can whenever they run low, or I notice.
I've done this for years, and find it makes a difference. Can't hurt to try it?
M.