Abnorm just like fiberglass except no mixing, no mess as everything cleans up with water. On that big a box I would use 2” foam. You can cut resting slots for rafters and I would recommend a sloped roof to insure water doesn’t sit or puddle as there will be some rises and dips on that large a surface. I would recommend not building bigger than the trailer as it complicates and makes build much heavier as well as the floor should be able to support several hundred pounds and you may need to change trailers someday. If you want a 77’ wide box get a few inches wider trailer. I usually build my floor with 1/2” plywood on top, ripped in half 2”x4” joists on 2’ centers, 1/4” plywood on the bottom, fill the hollow sections with spray foam and Poor Man Fiberglass the bottom. When the walls go on sitting on top the floor I lap the wall canvas around and underneath the floor. Some use the excess to form a drip rail to keep water from setting between the floor and trailer bed. I just after painting spray a thick coat of bed liner on the bottom. My guess would be dependent on how much support and paneling you use. Mine is 11’9”x80”x6’tall and I estimate It is less than 750 lbs. but I don’t have rafters and little wood support except at the seams where the interior doors are glued together. You will have to have ventilation as this method tends to make an air tight box.