Be sure to check out the tips on successfully printing a good, light Li'l Magnum!
NOTE: Don't try to buy an ancient mouse like the Corsair Katar Pro Wireless! I needed to support a different mouse to help me refactor the OpenSCAD source code for the Li'l Magnum!, and I have a few of these five year old mice on hand. There is no good battery mounting solution. I just soldered wires to a USB-C rechargeable AAA battery and taped that to the top of the mouse.
It might not be worth doing this mod even if you already own a Corsair Katar. You might be able to buy a VXE R1 SE for just a bit more than the price of a pack of rechargeable batteries, and you will have a lighter, lower latency, more modern and rechargeable mouse.
This is obviously inspired by Optimum Tech's Zeromouse and Zeromouse Blade, but using FDM and a more affordable donor mouse.
The thumb and finger rests are in a reasonable spot for me, and the buttons are lined up well.
My Li'l Magnum! with the Katar electronics, scroll wheel, and aftermarket USB-C battery installed weighs 26.13 grams.
The author marked this model as their own original creation.