Andy go back to the first page and look up the video I posted. Go to that guys you tube channel and watch his build videos.
He basically just screwed the whole thing together then coated it with fiberglass, it is heavy, but solid.
If I was building for rough off road I would do the skeleton 3/4 with foam filling the cut outs, then a 1/4 thick inside skin and a 1/2 outside skin covered in glass. Heavy but solid, you could honestly get away with 1/4 on the outside too.
Use a "cheaper" and lighter 3/4 ply and good birch ply for the skins.