I ordered one of the Tamron style pinch caps, for $1.68. If it doesn't fit well, I'll just make something. In fact, I already tried to make something, but failed. I went through the cupboards and tested every plastic lid I could find. Eventually, I ran across some of the old-school write protection rings from those huge IBM computers. My father used to run these things for the university in the 1970s, and I guess I had a few "toys" from my youth I never threw away. Anyone remember these?
Anyway, it's a perfect fit for the hood. I just have to stretch it the tiniest bit, so it fits snugly and stays in place. I tried to glue a plastic disc cut from a coffee can lid, but SuperGlue didn't hold. Both the lid (and I think the ring) are LDPE. I'm sure if I just did some research and found the right glue, I could fuse them together...