A fast, easy and cheap way to stop propane bottles rattling in the locker and leaving a rust ring is to cover the bottom lip of the bottles with a length of split small diameter hose. Hopefully the pictures are self explanatory but do use a softish hose. I found the perfect stuff at my local True Value hardware store, it's soft neoprene, is easy to cut and split down it's length, does not harden up like plastic pipe is prone to do and stays put once slipped over the lip on the bottom of the bottle. Total cost per bottle was less than $1.50 and has quieted down the rattle to nothing and no more rust marks. By the way the same technique works on older metal outboard gas tanks too.