We always uses 2x6 pressure treated lumber with outdoor carpet stapled to it. It worked well until it rotted out or the hardware rusted through... or both... about every 3-4 years. However, I have seen a few trailers here that look like they were made with different materials. I will be fixing up a trailer eventually and was curious what the options were. I'd like to make my bunks around 10" wide and 9' foot long with another cross support halfway between the end of the bunk and the winch.