I have designed a collar and I am making it out of stainless steel, it is to be riveted shut to be on my slave all the time.

It can be removed quickly if need be with Large bolt cutters LOL.

But this will look more like chunky jewelry than a ring collar or flat collar,
the rings can be slim and discreet and the flat are more put on men.

You can find almost an design of collar you imagine or your master might want.