There are two aspects - electrical and mechanical.
If you wanted to you could “use” a normal mouse’s insides for the electrical, though this wouldn’t get any custom features. Even using a mouse would probably require soldering.
For the body you could build it in a number of ways - plastic molding probably being the most expensive unless you’re making a bunch. I’d say that hand carved from wood or metal, or 3D printed would be the easiest for a one-off.
