dont put it in the bag, but use the straps and the flap of the bag to secure it inbetween the flap and bag. make the straps long and wrap the twice around the tubes, preferably with one through the bb, or bb/chainstay/chainstay bridge hole to make sure it doesn't slide side to side.
Any tips for securely carrying a bike frame with a messenger bag? Only has to go a few miles but would rather not have to get a bus.