Reiner Vogel built a lightweight 22" and saved a lot of weight by making the mirror and rocker boxes with a sandwich of thin ply and foam. Structurally makes a lot of sense to put the stiff thing on the outside to take bending stresses while the inside foam only needs to transfer shear stress from one skin to the other. Full thickness parts are needed where point loads impinge on the thin skins, can be wooden blocks.
edit: just checked his website and couldn't see anything re foam sandwich. Must have been someone else. anyway lots of lightweighting and EQ platform construction tips. A great website
http://www.reinervogel.net/index_e.html
When I rebuild my large dob (after finishing the figuring!) I hope to follow this path.