I've been looking at insulations like this one:
http://www.superhvac.com/astrofoil/help/
that have foil faces on 2 sides. There are a bunch of different
types of core materials. HVAC supply stores might have something cheap in bulk rolls.
http://www.quietride.com/article.html
has some pics of this type material.
1/8" sounds a little thin for plywood but might be OK if you found some high quality panels.
Alan P