Google "torsion box" and make the whole thing from plywood. Standard
3/4" stuff for the frame, and bending plywood for sheeting. The shape
can be constructed of one-piece plywood girders attached to
crossmembers, wich form the tosion box. The whole shebang then gets
sheeted with bending plywood, chreating a strong, relatively light
structure. Think "flying model airplane" or "plywood boat".

Forget the 2x4's and MDF. On another note, 2x4's are not properly
dried for furniture, so you'd have to store them an awfully long time
before you use them.

Bending plywood looks interesting and a possibility. Though, I was
hoping to accomplish my goal with commonly available material. Plus,
isn't the surface fairly rough? I would need something for the frame
though as the top span will deflect too much under weight. Is
hardwood a sufficient alternative to Pine in this case?