I made a few paper models to make sure the narrow width wouldn't throw the shape out of proportion. The final design is pretty close to the white model. (My wife didn't like the Benroy as much.)
The frame is 2x2s ripped from boards I got out of the dumpster at the end of my street. I lined the underside with cedar fence boards, and caulked up the gaps.
I had a bunch of R3 styrofoam laying around from another project, so I insulated the bottom with 1 1/2" of it. You can see where I doubled up some of the cross members to have something to attach the heavy duty 5" casters that will serve as a trailer.
The subfloor is 1/2" plywood from some ugly basement shelves that I tore out of our house when I bought it. So far I had $9 invested (in the fence boards).
I used 2 4'x4' pieces of 1/4" OSB and 2 2' x 4' pieces for the sides, because (you guessed it) I found them in a dumpster.
It's narrow! But I fit fine.
Once I got all the spars in, I did some tests to fit some 1/8" hardboard for my ceiling cover. I know from reading a lot that there are some folks who hate this stuff, but 20 years ago I helped build an outdoor skateboard park here in Michigan and we used hardboard for all the ramp surfaces. They sat out in the rain and snow and ice for a decade and a half before they moved the park, and that stuff held up great. I'll still waterproof it, of course, but I had a store credit at one of the box stores and so it was effectively free. (Are you noticing a theme here?

Tonight I framed out the interior cabinet, which will be a handy spot for books and such, and possibly a little compartment for a 12V system. We'll see.
That's as far as I am now, and I get about 20 minutes a day to work on tis with 2 small kids, but there's a foot of snow on the ground, so I guess I'm in no hurry.
-Matthew