I once made a backing plate out of thin aluminum sheet which mounted on the outside of the PVC and received the ply bolts. The PVC eventually cracked in a line even with the ply and then around the perimeter of the aluminum plate. I was able to fly quite a while in the cracked configuration, as the crack didn't proceed down the rear side of the ply and aluminum plates.

The PVC is really difficult to make resilient to vibration and impact. It's soft enough that reinforcements always seem to cause weak spots at the edges. An ideal reinforcement would taper the structural area as it goes to the edges. This is one of the reasons we eventually abandoned the whole PVC fuselage altogether and just went with a lighter flying wing design once we figured out how to mount the engine and servos to the wing. The other reason it that it's heavy for the strength it does have compared to other materials. It's chief benefits as I saw them was it was cheap, very easy to work, and easily replaceable -- much like other SPAD designs which utilize PVC parts.