While I wait for my screens and stuff to arrive I'm going to stick my three microcontrollers onto a breadboard and start trying to get them communicating and playing nicely.
I was tempted to try and shoehorn them straight into the case, but I want to see where the screen fits in. If it sits deep in the case I won't be able to fit my boards behind it (which is a bit of a bummer), but I have a backup plan.
I'm thinking of putting the pro micro behind the screen (with the micro usb port sticking out of the link cable hole), and fit the other two in the battery compartment. Rather than using AA/AAA batteries I'm going to use a LiPo, but I'm sure that can fit somewhere else :-)