Back before 'rocks,' I used to cut off the bulbous portion (maybe leaving a small portion of the stem too?-I forget) of a automobile tire valve and place it over the nipple, for dry-fire practice.
It's hardly noticeable, so you can leave it on, full time, except when shooting, if you like. If you have any concerns about unauthorized handling, in your absence, it's a small bit of insurance.
Good luck!!