Are there any safe add-on weights to add 2.5 to 5 pounds to a given hex dumbbell?

I have a small cast iron hex dumbbell set. I have sets of 15, 20, 25 and 40 pound weights. I’d like to extend them by being able to make the 25 pound 27.5 or 30 pound, and likewise the 40s into 42.5 or 45 lb.

Does anyone know of a safe (not going to fall off on the living room floor) add-on weight. I have seen ones with magnetic attachments, but I really don’t trust them to hold onto cast iron, as it isn’t all that magnetic. We have limited storage room, and I really don’t want to go out and buy4n or 5 more sets of dumbbells.