You've inspired me to get mine done. Bought the wood and tee-nuts this evening and will hopefully get this done in the next few days (modding a piece of luggage right now that was sagging all the time.
That's an AWESOME idea--I lost my BDM backrest pad on the highway riding two-up with my wife in a rainstorm. She didn't tell me that it was gone until we got home... LOL
Your idea works--especially using the rubber clamps to hold it to the sissy bar too.