love the concept & diy!
placement ideas....
- some kind of ergo hand rest, wouldn't need to move (3d printed?), i mean, if you're going to get creative with the device, don't shove your creation into a generic shell. or if you absolutely must use mass produced mouse shells, go vintage, bunch of them were more boxy, which would suit your components better. (modern mice....big arc in mouse shell, small trackballs, can't get boards close enough to surface..)
- there are a couple guys working on a fantastically interesting, extremely ergo mouse here on the forum. take a peek, hope it will inspire you, they are looking at everything about a mouse differently.
https://geekhack.org/index.php?topic=98360.0- aprox. what are the dimensions? could you fit both of them in between the alpha block & the numpad above the arrows? just thinking closer to center/home row might be better if possible. reaching over numpad might be awkward.
- love the thumb trackball idea, one for each thumb...
- with the recent explosion of split keyboards, individual modules (one for each half of keyboard) to be placed about the middle of the center edges of both halves would be cool, then you could use index fingers (and they are a little more accurate than thumbs....
- take a look at some of the keyboards that had small trackballs on them.
- the ultimate hacking keyboard is a split staggered with an optional thumb trackball module
good luck! & make sure to post back with progress.