I replaced my 4 bunks a few years ago on my old boat and trailer.
pushed the boat off at a public ramp, and tied it off on the dock, and drove up to the parking lot, and did the trailer bunk job.
think it was 2 X 6’s, that I had precut to length,
used an indoor/outdoor carpet that I got from a boat dealer,
layed the boards on the carper, folded it over on each side, think I used staples, and galvanized roofing nails to tack the carpet on.
added the trailer hardware, tighened it all up,
and backed it up, put the boat back on, and drove home.
worked great, and didn’t take too long.
good luck,
Jack