There don't seem to be any really "right" grommets for those valve covers. I walked down the nuts-bolts isle in the hardware and poked around until I came up with something that fit in the hole, with a hole in it for the stud AND with a larger top to sit on top of the cover. I put a stainless washer on top of that with the bolt and it works well.
Check the electrical grommets and the plumbing gromets. You should find something that will work.
