I also have a g3 aluminum boat. When I replaced my bunks, I used treated 2x4's. Bought a PVC 4x4 fence sleeve, ripped it in half, covered the bunks with it. 100% separation of boat/bunk board.
With the bottom of the board open, it can dry out quick, nothing to hold the water in. Hopefully I will not have to replace for a while.
The first couple of trips I was Leary of the boat sliding off the trailer when backing down the ramp. Have not experienced that.
I did paint the PVC black, to match the trailer.
I saw this entire idea on tinboats.com.