Hi. It is plastic soffit, sold in the UK for cladding. It is made of PVC I think. It comes in 2400 x 300 x 9mm sheets and slots together. It is also hollow, made in box sections. I'll see if I can get a photo. It is extremely strong for it's weight. When I'd finished the shell and before I put the floor in, when I pushed on the shell, the metal frame of the trailer would bend rather than the upper structure. It is waterproof and doesn't need painting. I have no idea how it will hold up to use.
Oh, and it's cheap!
I managed to get a rough weight and the whole trailer comes in at around 300kgs. It's having a test camp this weekend (tomorrow

) so I'll let you know how I get on.
I'd like to add a toilet and a sink inside, and I already have the brackets to add a shelf to the outside for cooking. I would also like to add a canopy for when it rains.