The front keel roller is fitted to give the jetski a bit more stability on the trailer and reduces the movement of the jetski on the roller beams/bunks when travelling on the road.
Source a suitable sized roller to match the shape of the bow keel. The roller is only required to support and guide the jetski, and does not need to be a tight or "wrap around the keel" fit.
Weld the roller socket central and flush with the top of the drawbar brace.
Fabricate 4 "U" bolt plates as per below using your supplied "U" bolts to determine the hole spacings and dimensions.