You don't need washers, or the retention pins, at all. Bear in mind the retention pins are not there to prevent the vibration isolators from separating from the mount, but there as a protective device to keep the entire camera from separating from the mount and/or aircraft during a crash. There are numerous ways to accomplish the same task, and the pictures below provide one of them. You can do just two corners or all four. All four is the best way.
Had to fit a replacement gimbal mount kit last night. The rubber dampers went on easily with the help of a length of dental floss would round one end and pulled through the housing. Fought for a while with the pins until I used some needle nose pliers to grab just below the bulbous area under the split. Holding the pin from there the washers were easily pushed on with fingers.