Originally Posted by Thurman
An underbelly that has had the covering removed whether through having to work in the underbelly, has been damaged, or whatever reason can be replaced with "Coroplast" sheeting found at Home Depot, Lowes, or most any building supply store. The factory stuff is a roll and they install it with the frame upside down which makes it easier than trying to do on your back.
Coroplast can be made easier to work with if you add "battens" at regular intervals. Kind of like how sailboat sails are stiffened at their edges.
Either strips of lath like used on old plaster walls or if there is enough room, strips of 1 X 2.
Lay the Coroplast out on the ground, measure bottom of trailer for good locations, slip the "battens" under and fasten with good Heavy Duty staples. Can also use 3/4" long wood screws with fender washers if you want it really secure.
This will take most of the sag out of the plastic.