I have always wanted to make one of these. But mine would be a little more bigger then your size. I've always wanted to make a life size version with really hard cardboard a some wood but i wouldn't have a place to put it.
I'd love to do something like that too; I've known some people who have made life-sized replicas of parts of the sets of M*A*S*H in their backyards, or at least replicas of the Swamp for them and their buddies to hang out in. If I did a life-sized replica of SST, you're right, there'd be nowhere to put it, but I would so definitely make it exactly as it was in the mid and late 80s, and I would feel so right at home.
What stuff did you use to make this?
Cardboard. Lots and lots of cardboard. Cardboard is your best friend when you make miniatures like this; I find the best cardboard to use is the kind that traditional postal parcels come in, or used to keep calendars stiff when they're packaged: it's not too thin, and not too thick, and it holds it shape better than thinner cardboard (like cereal boxes) or thicker curagated cardboard (like moving boxes). I've considered balsa wood, but it breaks too easily, which is why I've stuck with cardboard.
Other than that, I also try to get scaled items that I can actually use from Hobbytown USA, just to give it a little more look of realism - I've gotten oil barrels for Big Bird's nest area, crates for the side of 123, potted flowers and plants as well as pigeons for the roof of 123, among other things. I'd still love to find a couple of right-sized sets of tables and chairs to put in the arbor outside Hooper's, and give Big Bird his mailbox, but I've yet to find those.