>the horizontal axis need to be quite high (about 2.5') and I don't
want
>the structure to be too flimsy.
You might try some old interior doors. They are a sandwich of 1/8 ply, cardboard or foam core, and 1/8 play, with solid wood around the edges. Very light and strong. They make excellent desk tops. You can often get damaged ones for next to nothing.
Problem is if you cut them down you have to glue in another solid piece along the edge where you cut. A little tricky, but doable. I've done this to make doors narrower to use for shelves, but haven't tried Dob shaped pieces. It might turn out to be easier to get some "doorskin" 1/8 play and make your own sandwiched pieces.
Tom Clarke