Years ago I built a Phaeton 90 (same size)and had the same sort of problem. I ended up puttting sneakers on the roof of the car to act as chocks, put somthing to keep the plane level and tied/bungeed it down.

I dont know about the GP but the aluminum spars on the Rascal have withstood multiple assemblies and several strikes and the wood at the center has not failed. Id harden it with ca.