Anthony...
Most of us keep trying different things to make the better mousetrap, as far as plenums go. My first was from PVC. Expensive but good. Unfortunately when I went to a dual Z config, I had to remake. I had Extira in stock and used that. I also used a much more open design that allowed better flow. I made a few posts that showed how I did this, but didnt find the thread.

Although many like the stuff, I personally wouldnt use Extira for the bottom of a litterbox. It may work will for cut out parts that are sealed on all sides, but I have found that it swells and delaminates just about as much as regular mdf, and when placed in water, will almost swell to double its size. Many glues, except epoxy dont hold well and the dust is bordering on toxic. I have had it delaminate from everything I have tried to adhere it to, using different single and 2 part products.

This is not to say that you may hit on the correct combination, but to say that I havent.