Speaking as someone who has taken professional ESD protection training - Pink Poly Antistatic Foam is your friend. There is nothing safer to put your electronics on, and that $20 roll is enough to build like 20-40 mining rigs. Mine has a couple of sheets of the pink foam wrapped around a piece of cardboard (since the foam is pretty thin), with that entire construction zip tied to a metal shelf - perfectly safe, and works great.

Plastic is bad. Plastic builds up static. The same applies to making PVC rig frames - bad idea without a lot of extra work to ground it. That said, a piece of acrylic with hex standoffs (like you'd use in a case) makes for a wonderful MB tray, just don't let the MB actually touch the plastic. Wood or cardboard should be fine, although not quite the guarantee of a dedicated antistatic material.