Most diodes will do, I'm using the 1N4148 in the SMD package called SOD-123, though a through-hole package like the DO-35 would be easier if you plan to hand-solder.
The general rule of placement is between each key switch and the row of the matrix, pointing to the keyboard controller chip, but...
Yes, the diodes are a (very) popular addition to modern mechanical keyboards. They prevent what's called "ghosting", i.e. when say 5 characters show up when you press 4 keys at once. Totally optional.
Also yes, you can reuse the PCB, provided it has one. It's possible it has a membrane in...
More detail on the above:
KiCAD is an electronics/PCB design app. It does not care if it's a keyboard or a transformer you design.
The keyboard I've designed, as is pictured above, requires some more things to become fully functional:
1. Have the design sent off to a PCB maker to manufacture...
Here is the raw data for above layout on keyboard-layout-editor.com:
[{w:1.25},"±\n§","!\n1","@\n2","#\n3","$\n4","%\n5","^\n6","&\n7","*\n8","(\n9",")\n0","_\n-","+\n=",{w:1.75},"backspace"]...
Thanks everybody for the inspiration and ideas.
This is my (very preliminary and very sloppy) version of the flat machine so far:
It will be 3D printed in as few pieces as possible on my incoming large-format machine.
The screen will be Pimoroni's PIM431 10-inch 4:3 1024x768 with HDMI input...