I'm totally deaf in my right ear (have a Cochlear Implant for when I'm not shooting, & almost deaf in my left ear (have a hearing aid when I'm not shooting). I practice alone a fair amount & like to use a timer to measure improvement or lack thereof. My wife usually doesn't, so I have tried to work out how to use the timer since I can't hear it. I explored trying to add a light somehow to the timer so I could see it, but couldn't find a practical solution. I bought a ShotMaxx watch timer, but still have to hold it very close to my left ear, & even then, I mostly can't hear it.
Had a brainstorm the other day: I know pretty closely what my time to 1st shot is, so I hit the timer button on the table, I shoot, & then I subtract the time to 1st shot & substitute what I know it usually to be for me.
Not perfect, but a lot better than what I've been able to do previously. Hope this helps somebody. And, if you have found better solutions, love to hear them!