I needed another corded mouse and this time around I thought of @PKL@mastodon.social and @pronk@mastodon.social instead of Logitech's shareholders. These guys make open source mice among other open source hardware under the brand Ploopy. You can order one from them, assembled or as a kit, or you could print and build it entirely by yourself.
The mouse itself is pretty great. Coming from a long line of Logitech (MX518/G5/G500/G502), it's a bit larger than what I'm used to but I think I'm getting accustomed to it.
Here's another shot of it:
The PCB has electrical buttons permanently attached. The 3D printed parts have protrusions that touch the buttons.
You could possibly redesign the 3D printed parts only, but at some size increase you may need to start looking into redesigning the PCB.
The good news is that the mouse itself is bigger than the picture makes it seem. You might actually be OK without modifications.
If you do want to give it a try, I highly recommend buying the kit and fully assembling it yourself. That way you know that it works, and you have a solid foundation to apply the modifications to.
Good point. It measures at about 13x8.5cm.