The stock keycaps on my keyfob rubbed off the white labels due to wear. I can't see which is lock and which is unlock.
To replace them, I have created new keycaps with embossed icons on them, so they can't rub off. Additionally, I have set these new caps about 0.5mm lower than the stock keycaps, so that I am less likely to accidentally press the buttons when the keys are in my pocket or bag.
The 3D models for these keycaps are available for download on Thingiverse.
Make sure that any stringing, bulging or similar is cut off of the final model if present so that it does not catch and jam the button.