I asked a question a while back about how to fix a damaged luton box. I later discovered much of the box was rotten, so I totally resided the box. I was asked to make a post about how I did it, so here it is. Although not a pro finished, it is cost-effective and it worked out reasonably well. Obviously each coach builder makes their boxes differently, but hopefully this'll give you some guidance and the confidence to rebuild a box yourself.
The side of my van was plywood with some sort of fibreglass or polyester coating to make it waterproof. My plan was to reside with some sort of construction sheets and cover with fibre glass.
Although it's possible to fibreglass the side yourself, I think this is a pretty huge challenge. Not only is it a huge area to fibreglass, but also you're gonna have to smooth it out and fill holes etc with filler. So in the end I found this GRP sheet on ebay, which is used to create truck roofs. It's not completely smooth, and has a slight glass texture, but it's basically fibreglass on a roll, and fine for my purposes. I got a deal over the phone off ebay and got 14m x 2.5m for approx £400.
Next you have to get your grinder/drill out and get the old rivets out holding on the old side. I had 60 rivets per side, so opted for the grinder! Then you need to get your saw out and cut that old side apart. I managed to salvage the non rotten parts of this to reuse later in my project. Cutting this stuff is pretty nasty, as you get lots of fibreglass dust in the air. Get a respirator on and expect some chronic itching.
Next you need to create your replacement wooden side. I was using 9mm OSB for my build. This might be a bit thin for a free-standing side, but since this is for a camper and was to be backed with batons and insulation, it didn't have to be particularly rigid. I would suggest 11mm minimum if you're rebuilding a standard work van, and the sides that came off were a about 11 or 12mm.
With the OSB side made to fit, I removed it and reassembled on the driveway. This is my template for the GRP.
Then I laid the GRP over the top and cut to side to match the OSB. I also drilled holes in the GRP to match the OSB holes I'd made earlier when attaching the OSB to the van. Again, defo need a respirator and protective clothing if you want to avoid chronic fibreglass dust itching.