The first step in the design is to measure the bottle. The shapes of bottles vary pretty widely, but the caps seem to be totally standard. Because of that I decided to make the lock clamp onto the bottle cap, and left some wiggle room for the size of the neck.

I made the rough shape of the bottle, then designed the lock to be a clamp with a single 4-40 screw as the hinge part. The other end of the clamp has a hole that fits an off-the-shelf luggage padlock I found in a 6-pack at a hardware store.

I used Fusion 360 for the design because it's a great solid modeling tool. It has mechanical assembly functions that let me test out the motion and figure out if it would easily come on and off of the bottle. Fusion 360 is free for students and hobbyists, and there's a ton of educational support on it. If you want to learn to 3D model the kind of work I do, I think this is the best choice on the market. Click the links below to sign up:

Student/Educator

Hobbyist/Startup