3 methods. In the pic below I made specific diameter and length plugs from foam (pipe insulation) skinned with Gorilla Tape. I also bought little rubber balls at Dollar Store & rubber plugs (like test tube stoppers) at The Depot. To close up around the uneven holes for running boards mounting I jammed foam in there. I think for the round holes you could just put Gorilla Tape over it. If the metal is clean and it adheres, I think that would last a year or more. That's what Ford does on pickups, puts tape over it but it falls off eventually. That's where I got the idea from.