What I did for my crossbow is purchase the U-Clamp style gun holders (a pair) which clamp onto the bike rack. You know the ones that have rubberized feather feelers on the inside of the U-Clamp which grip the stock and also cussion it. Worked great but I did not use it much for the crossbow on a bike, they are actually now mounted onto my treestands to hold the bow while in the stand.

BTW, the holders I got only cost $30 and are redally available at most outdoor stores.

Hope this helps you.