Take a look at Bill's articles on ez.org ("Those EZ Chronicles" link on the left). The one named "Plenty of Plenums" describes the materials he used. I assumed he found some exotic material in the dumpsters at Lockheed but it turned out to be 3 layers of BID and EZ Poxy plus a few post cure cycles. Maybe a little high temp BBQ paint to make it look exotic?
Haven't seen/heard anything about adapting the idea to an updraft config, though.