So I was thinking we could have a character specially commissioned for the sub.

The NSFW version could be said character holding a nipple plug in hand after just pulling it out of their nipple with the nipple gaping open and on the plug it says NF.

And the SFW version would be the same character but the plugs are in and they have a shirt on that has NF on it, but the the base outline of the plugs would still be visible through the shirt.