I used a rubber shim. Don't remember if it came with the lock. It's about 1/16" thick. You could use an old tube but you'd have to fold it a couple times to get it thick enough. The bracket needs to be tight enough so as not to slip, but loose enough to allow you to rotate the bracket on the post for install/removal since it needs to be folded back to be in line with the bike so your leg doesn't rub on it....not against the frame, but close.

by the way...nice marmot