Well since I work at a lumberyard and sell this stuff on a regular basis, I'll fill you in on the good stuff.
Correct Deck (which is now called Siesta since GAF bought them out), TREX, Fiberon, Timbertech, ChoiceDek are all good choices for turning. They are a plastic that is impregnated with wood fibers. Some like Correct Deck will have more wood fibers than others. There are some composites out there that are more of an extruded product that is almost foam like such as AZEK. It is a lot lighter but will tear out a lot more. As far as dimensions, I haven't really had to change all that much to get the same sound. The call itself weighs a ton, but sound good with either slate or ceramic. I've made one with aluminum and it's a screamer for sure. My thoughts on the material is that it's kind of like using acrylic in duck calls. The harder denser composite material allows a cleaner sound. But that could just be me. And yes it does create a crazy mess, but for the most part you can pick up a whole pile of it up at a time when cleaning. I also burnish it after I am finished turning it to try and bring back some of the plastic color to it. I've been meaning to try some Mag Wheel polish on it maybe even a slight flame to see if you can get a good sheen to it. KC--Since the Correct Deck has more wood fibers I was thinking of trying to get a finish on it (maybe Tru-oil) and wipe off what the wood fibers don't soak up.
I'm pretty much out of samples at work right now
(Have no idea where those went!!)) But when i get some in let me know if you want a piece or two and maybe we can work something out. I'll get some pics up tonight with the pots that I've been turning lately. Whew, hope that wasn't too much for you all.