I wanted a rear wall on the van as the shutter is ugly, noisy and drafty. It also gives me a semi outdoor space for storage of things like the gas. I made a wooden frame of 25mm thick wood and filled the gaps with 25mm celotex insulation board. I made the door out of wood frame and celotex too.
Below: Door is halfway there, I decided on a stable door so we could have it half open for air in the summer without losing privacy. I made a storage box that will be the sofa when cushions are added. There are 2 sheets of OSB on top that are double hinged so in normal use they make the sofa but it can be turned into a second bed
I purchased a foam double mattress and some fabric from ebay and had a seamstress make up the cushions to the sizes I wanted. The cushions are a sofa by day and when needed a mattress for the second bed by night.