This is a snap on adapter for a common type of inexpensive bicycle bell (sold under various brands, for example: https://www.amazon.com/Accmor-Classic-Aluminum-Bicycle-Adults/dp/B0CR5C7LLK, https://www.amazon.com/Xthuge-Classic-Aluminum-Bicycle-Black-Left/dp/B0C1HLTB3K/138-0267660-6916137) which allows it to be operated remotely via a standard shifter cable, using a dropper post lever or similar actuator.

Why did I make this? To solve problems with two of my bikes:

• My winter commuter bike, with pogies/bar mitts: Can't ring a standard bell from under there, but this works great.

• My cargo bike with crowded handlebars: Not enough space to mount a full sized bell anywhere near my thumb, but there's room for a dropper post lever!
  

Probably you can come up with other ideas!

The bell it's designed for comes in left hand and right hand versions - print the model for the bell you have. The adapter should fit on with a tight friction fit. Glue can be used, but probably isn't needed.

Drill or melt a small hole into the bell thumb lever to put the cable through (see pictures). I found that placing the hole closer to the center of the bell worked best (you can see two hole positions I tried).

I printed in PETG, but probably most materials will work - it's not under a lot of strain.
Recommend to print with support, at least under the tab on the opposite side to the cable, as it has some clearance for the bell's spring.

The adapter is designed for a shifter, not a brake cable. Brake cables are thicker and will not work with this model (but it could be adapted for that).

Any dropper lever should work. Even a 3D printed one, as the forces involved in ringing the bell are very small.