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I don’t know quite what your asking cubic interior space, interior floor space ??? I’ve several trailers in different sizes, so I’ll go with the 4’x8’ in the article… with 2x4” studs interior will start with 96”L-8”=88”, 48” W-8”=40”, 88”x40”=3520sq/in➗144 sq/in=24.4sq/ft , best I can do, hope that helps, Shalom!
I used liquid truck bed liner as my exterior final finish, it’s cheap ,easy to handle & quick to install..my 3/4” ply floor was coated w/ 5 coats of bed liner that faced the road, ( buy it by the gallon)..after I built the box body, I rolled 5 coats on outside roof & walls before I mounted door , Windows, power vent & roof rack. I used 2x3” wall/ceiling studs w/ 1/2” ply ext walls & ceiling, where ply edges meet, I applied rubber tape to seal , capping with aluminum angle, my reason is that water capilllaries thru edge joints left unprotected & unsealed, i’ve seen plywood campers built without edge protectection (paint only) that were used & stored in the weather 100% of the time,replaced roofs due to leaks/rot in a short time. Good build, Good Luck!