A decade ago I used to walk around homedepot knocking on every material in the place, looking for dead materials for the latest diy speaker project. Corian, PVC, ply, etc. My kids were young so they would come shopping with me, and when we traveled on vacation I used to tease my kids that we would stop at HD to shop for "speaker wood" and they would howl in protest - so funny!
I once built a big planet10 horn speaker out of Trex decking. Trex back then was 4/4 thick, polyethylene mixed with sawdust. I glued it together with West Systems epoxy. It seemed totally dead as a long single plank, but built into a speaker with smaller spans the resonance became audible. They were by far the worst sounding speakers I have ever heard, even tho same speaker made of BB was among the best. Fortunately the epoxy failed within a week (nothing can glue PE) so they ended up in the dump quickly, and it took it two people to lift it into the dumpster! It was lots of fun. Gotta do a DIY again soon.