Need to use a flat bunk trailer to carry a stepped hull boat across the country. Single trip use. Any ideas?

Screw 2x4s or whatever thickness needed to the top of the existing bunks. Be sure to countersink the screws below the surface of the wood at least 1/8". Staple some carpet over the top assuming the boat will be lifted on/off.

Yep...usually 1 2x? Per step is how we set up lifts usually