If anyone wasn't already convinced that 3D printing is on a roll, they may change their tune after they see what Pyott Design did.
Long story short, Pyott Design created the Statial, a 3D-printed replacement for a mouse's normal top cover.
Just make one and install it on your mouse and you'll be able to adjust it so that it fits your grip more comfortably than most, if not every, pre-designed peripheral.
Statial costs $70 / €53.36 and is made for the Logitech M100, which, ironically, costs only $10 / €7.60.
Then again, if you have your own 3D Printer (by some miracle) and somehow get a hold of the plans, you can print your own Statial instead of going here for it.
No doubt, Statials for other, more intricate mice will be made eventually.