I've used a sealed MDF plenum under a sacrificial spoilboard for ~8 years, no problems. And while I don't have experience with plastic plenums, I agree with Eval that HDPE has HUGE thermal movement. I've used sheets of it for other projects.

As for gluing to a non-porous material, I'd highly recommend a melamine glue (ie: Wilsonart, RooGlue, etc...).