I thought you only wanted them for a HC so just install, use, remove, no?
I can't find a link to the ones I have. They existed when I was still in Oz and I bought another set in the UK many years ago. I think they still have the original button cells in them!
I've still got a few of those little single-LED flashers. I can't remember the brand. Presume they're still around.
Similar to these:
https://hollandbikeshop.com/en-gb/bicycle-lights/ikzi-bicycle-lights/ikzi-bicycle-light-set/ikzi-bicycle-light-set-mini-1-led-elastic/
Otherwise I'd get a couple of those £2 service station jobbies.