The open source / open hardware offerings from Ploopy are very attractive for people that don't want to take this route
https://ploopy.co
They've apparently suspended shipping to the US, though. Not sure who to blame for that one..
I was using a vertical Logitech mouse but was still experiencing discomfort mousing out to the side of my enormous Advantage2 keyboard. In an effort to head off any compounding RSI issues, I switched to a ploopy Adept trackball. Had no idea if I'd like switching to a trackball, and I love it! My hand never moves in an awkward motion, and mousing has become pretty restful. Better be careful tossing the ball up in the air idly during meetings as I won't be able to get a replacement if ploopy has paused shipping to the US (It's a pretty dumb habit at any rate).