A rip-off obviously, but postage is included.
I haven't been able to try this though I've had it for a month or two, but someone else may wish to play with one.
The hole in the middle is 20mm. It's intended to be held on to a phone by the lens, which screws through it into the phone's clip. Only the wide-angles do that, not the "macro" lens.
It's easy to hold on to common objectives where the outer barrel unscrews, with a couple of rubber "O" rings.
Ebay searches will mostly return versions which are a larger LED ring, or a type with no diffuser.
Try item numbers
"Selfie LED Ring Flash Fill Light Clip Camera For Phone iPhone Samsung Tablet PC"
The internal thread is 52mm, but unfortunately 0.5mm pitch so doesn't fit filter threads.
Have questions about the equipment used for macro- or micro- photography? Post those questions in this forum.