Tim,

Yes, that's definitely them; thanks so much for that. What search terms did
you use to track them down? Or were you already familiar with them? It's
interesting that the use I thought they could be put to seems actually to be
what they're designed to do.

As you've undoubtedly seen (and thanks for checking) Amazon UK lists them,
but as currently unavailable. And actually, when you follow the link,
Amazon US says exactly the same.