This may be considered off topic, although I actually feel it isn't in this case. But have you considered adjusting the trailer so your boat sits further ahead, and the bunk boards are actually out past the stern slightly. Every boat and Jetski I've purchased, left me disappointed with how they sat on their trailer. All 4 were adjusted by me, till their bunk boards extended out behind the stern, which to my thinking offers support for the back. I simply loosened the winch tower, and the bunk board pivots, so they can self adjust to the hull angle, pushed the boat or jetski ahead till I had about a half Inch of the boards behind the hull, then tightened up everything I had loosened, and tied them back down. They spend so much time sitting on their trailers, or bouncing down the road, with all that weight hanging off the back, it makes me cringe to not support the part of the boat that needs it most in my opinion.
I've done the same to many friends and relatives trailers over the years.