A lot of the newer trailers have a single sheet of black coroplast material as the underbelly. I just had to pull mine off for a frame repair. I'm trying to find the roll material to replace it, but may go with the 4x8' sheets if I can't find the roll.
My thought is if I go sheet, to run a series of 4' wide strips down the center and fill in each side with smaller panels using a 1/8" thick steel 2" wide strap between the cross joists and at the joints of the side panels. Same thing in any area where I might have to access something for the future. My thinking is that way I don't have to take the whole thing off later if I need to repair an electrical line, tank fitting, etc....
