When I asked about the 'gasket' above, I was actually referring to the spacer, my bad. Then reading back up, OP mentioned this thread, where it was clearly listed that the spacer was made from HPDE:
Originally Posted by Isfahan
Parts I used:
1 12x 12 sheet of HPDE of ¾” thickness (Polyethylene HDPE LDPE Sheet | Industrial Plastic Supply, Inc.)
2 front tophats from an NX2000. I’ve heard any B13 would do, but I can’t confirm that.
6 grade 8 2” 5/16th bolts, with corresponding nuts, washers, and lock washers
Parts I used:
1 12x 12 sheet of HPDE of ¾” thickness (Polyethylene HDPE LDPE Sheet | Industrial Plastic Supply, Inc.)
2 front tophats from an NX2000. I’ve heard any B13 would do, but I can’t confirm that.
6 grade 8 2” 5/16th bolts, with corresponding nuts, washers, and lock washers
As for gasket, usually those are just paper, so I can't imagine those being too exotic like Corian?