I had to rebuild the door to my pop-up Sunlight and used whatever good pieces of scrap that was in the garage. The problem you will have is fastening the pieces together. The manufacturer used large staples to allow the joined pieces to flex when the unit was under torque stress form rough roads that twisted the cargo box. My door had sagged and was rubbing the sill. I used 2x2 steel angels at the corners to maintain 90 degree angles. If weight is not a problem you might consider using larger pieces for greater strength.