In a bid to make the overall dimensions smaller and also easier to build I have built the Mark III which is based on an Arduino Uno board, which is more expensive but a more reliable USB port and a built in DC power in port.
The button board is still the same but there is a slight change in the design of the filter board.
I may have left off one extra ground pin on the design.
Because of how the Uno is built you can connect everything directly to the board using female to male dupont wires. I was lucky enough to have an Uno board with both male and female headers so I had even more flexibility.
And heres the final build.