Coroplast is an option, you can get it at any plastic supply distributor. It is light and inexpensive. It is basically the plastic equivalent to corrugated cardboard, exterior skins with webbing in between. It comes in many colors and thicknesses, 6 or 8 mm would be best for spanning the width of a trailer. Some manufacturers use thinner, black Coroplast as their underbelly material.
You can also purchase 'H' trim with a center open space, kind of a double 'H' to secure the long edges to prevent wind problems when towing. Screws thru the open center into framing underneath with keep everything in place. Greenhouse places also sell this stuff for holding the plastic panels.
Some yrs ago I used these items to replace badly torn underbelly fabric with something more durable.
Or you can use FRP (fiberglas reinforced panel) or Adzel panels, but Coroplast will stand up to rocks and other flying debris better.
