HP-21 calculator restoration/Arduino-fication

I made a 3D CAD model of the rough dimensions of the case front in SketchUp, to make sure the components all fit well:

The hatched part is the boundary of the interior space; the brown boxes are exclusion zones for the screw receptacles that stick up. The gray and yellow thing is a battery; any of the reasonably capacious batteries from Adafruit and SparkFun will fit. The white thing is the SparkFun SX1509 breakout board I'm going to use to control the display (3 SparkFun bubble displays) and read the keypad.