This piece is to be glued on the bottom of a glass microwave oven plate that doesn't have the little knobs to rotate the plate
(remember, a microwave oven that doesn't rotate isn't a real microwave oven, as once said Thorin Oakenshield)
This is in case you found a compatible plate (pawn shop or else) that doesn't have the knobs underneath. You can glue this on the underside. Use microwave-safe epoxy, not CA glue/super glue.
Edit : One of these was installed in my microwave oven and melted after I made some microwave popcorn... You might have to consider this, or printing a bunch, replacing them after a while...
An article on the making of this piece can be found on my website
These settings don't really matter, really. I used white PLA though