They come as pairs. I bought some aluminium to make my own, I need 6x9, 6x17 and 7x5 for my De Vere 5108. I've made them in the past for a Durst, and a Johnsons V45, The 35mm inserts for the Durst M601 were the trickiest due to their small size.
You could use plastic or even card, my cardboard 6x17 inserts must be over a decade old and work fine. Mounting card impregnated with wood hardener (similar to super glue) is rigid.