Plastic decking is used for bunks pretty often (the 2x4 or 2x6 synthetic lumber - usually HDPE.) Be aware that the stuff is less rigid than lumber - you may need to increase dimensions or add bunk support if you go this route.
As you said, the HDPE screw-on pads will work as well, but will also concentrate the force as they're usually narrower than the bunks.
On my SIB, I actually preferred having to float the boat off - it was less concerning than dragging the boat across the bunks. Had to go a little deeper, but the trailer was getting wet anyway, so...