There are tutorials out there but none list the essential skills necessary to build-it-yourself from scratch such as casting and molding. If there are pre-existing software or electronics out there to reduce effort, all the better. I see it as a hobby over a year or two. I have the idea and designs but no skills!
There are two aspects - electrical and mechanical.<p>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.<p>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.