Well if you are building a plinth, I really don't think you can do any better than a BDR Source Shelf. Even the thinner Shelf would be better than pretty much anything else you could find. I built a few and nothing came even close to the Source Shelf. The stuff is just so damn stiff and dense and highly damped- perfect for a plinth!
Don't be put off by it being carbon fiber. There is really only one layer of carbon fiber on each side. As far as cutting, shaping, machining, sanding, polishing, it works just like wood. Very, very hard wood. Not as hard as aluminum. But I have some very dense hardwood in my shop that is actually harder to cut and sand. Very doable.
Mine was cut first on a band saw, then hand sanded. Other parts were routered, drilled, and cut and threaded just like any other material.Ā
If you decide to try it first wrap the whole Shelf in blue painters tape. Leave the tape in place all the way to the end. That way only the areas you work get exposed and you wind up with a perfect blemish-free finish.
