Just starting out after decades since last bike ownership. New woman's cruiser bike and bought Bontragger rear rack (25LB load) that attaches to seat post as I will not be carrying much weight in the bag and easier for me to install or so I thought. Seat post is too small and the shims that came with the rack are too big and rack clamp will not tighten enough on the seat post to keep rack above the tire. Any suggestions as to what else to try? My husband thought about cutting the shim to make it smaller. I thought about rubber pipe insulation tubing. Getting frustrated and appreciate suggestions.