Anyway I guess they're in the mail. In the meantime I was looking at DIY center caps, and considered using "conduit caps" but none happened to be the right size, so I was trying to think of something else when my dog dropped a racquetball next to me, and I thought "sure, why not"...
and since they fit I ordered some black racquetballs in case I lose my center caps (or don't like them as much).
Oops, I just noticed that I put my rear tires on
backwards, so I'm glad I took that picture!