I've been slowly working on this as well.  I've been painting the interior
sheet metal with Kool Seal Roof Coating (white stuff) (also put several coats
on the exterior roof under the luggage rack -- figured reflecting some heat
sure couldn't hurt).  I've also been filling the non-vent, non-wire run
channels with low expansion foam.  Haven't gotten all that much done -- side
door, rear hatch, tops of the front doors (foam only) and area above the
driver and passenger but the results are pretty dramatic so far.  McMaster
carries an asphalt roofing sheet material for about $13 usd for a 29" x 54"
sheet (don't quote me on exact size) that I intend to use on the floor and
drivers side wall when I tear into it later this summer.

Chuck
'85 Wolfsburg Westy -- 'Roland the Road Buffalo'