Jim, a friend of mine in the FGRS (Florida), built a decent size 3-level indoor layout, in an industrial condo with a/c.
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Don't make any assumptions about materials until you check your local Home Depot/Lowes. Jim found that regular 4x2 studs were much less expensive than anything else [figures - they use a lot in each house.]
I don't know what he used for the surface, but I'm sure I can ask. I could even put you in touch so you can ask him directly what he did? (P.S. He didn't use 'hardboard' as it is almost unknown here in the US.)
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