I think I've worked out what my next build-it-yourself project will be...a replacement to a mouse that can be used with fingernails instead of having to hold it. I have an auto-immune condition (Sjogren's Syndrome) that is causing the skin of my hands and feet/legs to burn/sting fairly badly (the Sjogren's is causing Small Fibre Neuropathy). My hands were hurting enough by the end of the day today at work that I couldn't really hold my mouse anymore. Pressure on the skin - particularly against ridged objects like the scroll wheel - aggravates the Neuropathy issues...and between the palm holding the mouse to move it and the scroll wheel and buttons on the mouse poking my fingers it becomes very hard to use the mouse when my skin is feeling like I've skidded on my hands across asphalt. My hands are not like that all the time...yet...but I can see a future when they will be, so I've decided to design/build an alternative device for me to use instead. I've found that typing on my fingernails doesn't trigger/aggravate the pain in the skin of my hands, so I'll design a replacement device for a mouse around being able to use my nails as the primary input device instead.
A thumb-ball trackball wouldn't really work either - you still use the fleshy pads of your fingers/thumb to move the ball, press buttons, etc, and to move the scroll wheel if it has one. It likely wouldn't aggravate my palms as much, but that's about all I could say would be positive about it. A palm-ball variant would be even worse.
So I'm thinking I'll build a box with 5 keys in a cross-shape. Left/middle/right click across the horizontal, with scroll-up above the cross and scroll-down below it. Behind those on a 90-120 degree angle I'll mount a resistive touch sensor - resistive so that it reacts to fingernails - to use to replace the "ball"/movement part of a mouse. About the only thing it wouldn't do easily would be click-and-drag...but that would be more a case of learning alternative finger use/positions than anything else. Or I could use another key as a "toggle left-click"...i.e. it stays "clicked" until pressed a second time. I'll have to think a bit about that...it's an easy solution to the issue.
I know where I can get the keys and keycaps easily, and the controller to use to turn those into a device that any PC will recognise as a USB mouse. I think I could probably use an instrument case (the kind with the angled end for a LCD to be mounted in it) as the box. I just need to find the resistive touch sensor to put on the angled back (there's a lot of cheap ones pre-mounted on TFT screens, but I don't need a screen for a mouse).