I have no personal experience with it, but you might be interested in this completely open source mouse:

https://ploopy.co/mouse/

https://github.com/ploopyco/mouse

It looks like you can buy a fully assembled mouse, a build it yourself kit, or do it from scratch with the schematics. Doesn't come with quite as many side buttons though

Try the Statial B. You can adjust it to meet almost any shape you need.

Note, it can be printed with an FDM, but it'll need a 0.2mm nozzle and really good tuning. Some of the parts were a pain to print for me

https://github.com/PyottDesign/Statial-b

My main complaint is that the Statial B only has 2 thumb buttons, and I'd prefer to have at least 4. I've considered modifying the design of the relevant plate, but there's still the issue of not having ports to wire the switches to the raspberry pi it runs on.

I also have a ploopy mouse like the other commenter mentioned. It's definitely not a replacement for a G602 (that's also the mouse I used before switching to open source), but it gets the job done. I just need to buck up and finish assembling the Statial B