Cut these out with a laser cutter. I would recommend acrylic plastic between 3mm and 6mm thick.
Use the .dxf file if you just want to use it unmodified. If you want to make dimensional changes, or make more/less, use the .scad file and OpenSCAD.
Note: The slots are slightly different sizes (the Adafruit PCB is slightly thicker than the Pi PCB), so you may need to flip it around if it doesn't fit the first time.