Here's a fantastic slideshow of a commercial quality homemade slide-in truck camper handcrafted with plywood framing from 'spidersfrommars'.
The frame is constructed from 1/2" birch plywood, with ample large holes cut out to decrease its weight. A layer of 1" styrofoam is applied, and aluminum sheeting covers it all.
It also features a lightweight curved roof and built-in furnishings.
(Note: The video oddly cuts out halfway through promising more to come.)