I agree, this is the way to go..I've got the same bag, and had the same issues with straps hitting the bodywork. Tried some heavy clear soft plastic to keep it from rubbing - flapped in the wind and messed up the paint worse!
I found that removing the rear seat and attaching the bag directly to the seat worked better. I adjusted the bungies so the hooks could all be hooked together snugly under the rear seat pan and then re-attached the seat.
Worked a lot better for me.
I have a joe rocket tail bag and when I first got it I thought the velcro straps were made too short so added more length to it with a couple more strips of velcro so it would attach under the rear tail.
.then one day I realized the short straps were designed to hook under the rear seat..not under the tail...so now I just pop the seat..hook it. re-attch seat..It is actually more stable cuz the seat being closed down helps hold it even tighter