Depending on the size of the chain, put a cut up piece of bicycle inner tube and place it around the chain (where it'll touch the bike). If its real tight going on, some soapy water (like a tire change) will help. No mods to the bike and cheap...
You can just see on the trailer I built, atop the right side rack, a piece of chain covered with a bicycle tube. One side is welded to the rack, the other side has a link welded to the rack and and lock secures the chain thru the front wheel. You can see the left side rack, the covered chain is hanging.