After the original version was finished, I submitted the shield in a contest organised by www.ledsee.com. It took first place, which means it is now offered through their webshop :) Therefore, a cleaned-up SMD version was made, with better buttons and bigger PCB:
Because the button layout is slightly different, the code to read out buttons is changed. Updated versions of the manual and example code is at the bottom of this page, or available via Github