One other material to consider is a close cousin to acrylic---Delrin. That's a brand name I believe, not the material's generic name. Delrin is the material VPI used for the platters of the HW-19 MK.2-4, Aries 1 (top layer), and TNT (pre-Mk.6) tables. It is a little denser and harder than acrylic, and has a matte sheen in contrast to the high gloss of acrylic. I had an acrylic arm board for my Aries 1 made at a good plastics supply house, and the price for a Delrin version was only slightly higher than for the acrylic.
Another option to consider is to have two 1/2" thick boards made, with a thin piece of constrained layer damping placed between them, for damping of any ringing in the material.