Quartz crystal gasket, isolator, platform or whatever
It seems they changed the design so it looks different, but everyone still stocks the BIVAR parts.

https://www.digikey.com/products/en/crystals-oscillators-resonators/accessories/175?k=&amp;pkeyword=&amp;sv=0&amp;pv1989=0&amp;sf=0&amp;FV=-8|175%2Cii2|2245&amp;quantity=&amp;ColumnSort=0&amp;page=1&amp;stock=1&amp;pageSize=25