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I would go 5x7. Gives you a little more room. If you can get your hands on some billboard sign facing it works great really light and strong.
That's what I have. Just got 6 sheets of aluminium faced billboard sign facing . 4x4 runners with 2x2 joists,3/8"/1.5"styrofoam/3/8" plywood sandwich for the floor, 2x2 studs with 1.5" styro between, coroplastic insideand aluminium insulated roof (have one 4x8 alu sheet from scrap). Two plexi windows. That's the plan
What the black skis made out of?MD