Just get a sheet of silicone (like the baking kind) and cut your own?
If you look at that lid, there's a small lip just inside the edge that centers the lid in the opening. I actually do have a silicone mat I could use, but I don't think I can cut a 6" circle out of it that would work.
Sudden thought; I wonder if I could invert the lid and glue silicone to that side. No lip, if I remember right.