Hmm... how often will you be using the mouse buttons as "mouse buttons"?
Perhaps if you had your 3 mouse buttons above the trackball,
you could map your upper 3 buttons (to the right of the 4 way) through
them and eliminate a set of buttons.. in other words just drop the 4-way
buttonset down a few inches and use them as you mouse buttons..
or even eliminate both sets of buttons and just use the 1,2,3 of the 8-ways
buttons as both the mouse buttons and the 4 way buttons..:-)
(not saying you did, but sometimes people get a little button happy
and end up with a really complex looking panel when less would do the
same amount of work.. in the end, it'll be whatever you are
comfortable with..get some cardboard and cut it to you panel
dimensions and play with placement - you'll soon find what you like and
what you don't :-)