@Ti7z:
Exactly what osumkeyboards mentioned.
There are a couple useful tools out there. I usually start by drawing my layout using:
http://www.keyboard-layout-editor.com/
Once I confirm the layout is supported by my PCB and case, I will grab the raw data and and pop it to http://builder.swillkb.com/
This site will get the CAD file generated. Usually a DXF file if you are using a laser or water jet cutting service. I have used STL files for 3D printing, if that’s what you prefer.
If you are using a laser/water cutting service, the thickness of the plate is defined when negotiating with the vendor. 4mm should work, but be mindful of the pockets of each switch.
Hope this helps and welcome to Keebtalk!