HK Series Industrial Interchange Series B Dust Plug, -06 Coupling

For sustained heat in that range, EPDM is your best fit here. It holds up from -65F all the way to +300F, which puts your 350-degree operation just at the edge. If you're regularly hitting or exceeding that, FKM is the stronger call since it handles up to +400F and has a low end of -15F. Buna-N tops out at 250F and Neoprene at 212F, so either of those would be getting cooked at your temps. FKM gives you the most headroom on the high end.