Key Findings

[mivok]

It might be worth looking to the mechanical keyboard world to see if there's anything there that would work. Quite a few companies will do completely customized computer keyboard keys with small order sizes, and I found the following company while looking at the deskauthority wiki which claims to be able to do custom doubleshot keys of different sizes too: http://keycapsdirect.com/customkeycaps.php. One thing I haven't found yet is smaller than normal (where normal is a PC keyboard) keyswitches. I suspect if you could get small keyswitches, a pcb manufactured, add an ARM microcontroller and LCD display you could have a completely custom calculator for less than you would think.