Lemon works as well. It’s not as hard as a ball gag, but she is not going to want to bite through it anyways. As with the egg above, it will show if it has been bitten into and punishment can result from that (more than just the sour). Having just tape over the mouth will sometimes separate and lose it stickiness. This is normally due to the fact that the jaws are still mobile. If you combine something like the lemon and then tape over that, the lemon will reduce the mobility of the jaws putting less strain on the tape. Also with the lemon, once your submissive gets better at not putting teeth prints in it, you can use a grater and grate part of the rind off, making it softer and easier to puncture with their teeth.