BrannyThere are only 9 switches. You can hand wire, don't need a PCB. You don't even need a matrix. I did this with the 6 key version, and plan to do it with this one as well.
theimmcCan you elaborate for the newbs? What is that component called and where did you purchase it? I see what you've done in terms of wiring and drilling the hole, but can't figure out any more than that. Is that board programmable?
baechooboiThat microcontroller is a Pro Micro clone, available from China for about $3 to $4 shipped (depending on where you get it from). The original is made by SparkFun, who sells theirs for about $20. At that price, you might as well buy a Teensy 2.0. It is essentially an Arduino Leonardo in a small form factor and can be programmed with the Arduino IDE. I buy mine from AliExpress. Get the 5V version, and do not get a Pro Mini.
Since Arduino already has a keyboard class that does all the heavy USB lifting, I use it to make the Pro Micro appear to be a keyboard when it starts up. After that, I just need to sense keypress, and send that key over USB to the host. You can see my source code here: https://github.com/theimmc/six-key-pad
I wired one pin of each switch to an input pin, and the other pin to ground.
The code was originally written 2 years ago for an earlier version of the Arduino IDE. It may need some updating. I'll update it when I receive my tester later this week.
By the way, it was a real pain to open up the old case, not sure about this one. I highly recommend getting a spudger set, usually sold as tools for opening up phones - you can get a cheap set for a couple of dollars. You can use screwdrivers, spatula, butter knives, but you risk scratching the case and / or hurting yourself if you're clumsy like me.