Print with 0.2mm layer height. Default top/bottom layers and infill will work. No supports are required. The author used the "0.20mm Speed" profile in prusaslicer.

If you want to add some color to the lettering, you can switch filament at 0.6mm and again at 1.0mm. Here's how that should look in Layers view when viewed from the Bottom.

Next, it's time to place the heat-set inserts in the raised portion of the print. Begin by pressing the narrow end of the insert into the plastic by hand. Once the bottom of the insert is in place, heat your soldering iron and have a pair of needle nose pliers handy. Put the soldering iron into the heat-set insert and push down SLOWLY as the plastic melts. Once the insert is flush with the plastic surface, place the pliers on top of the insert to keep it from pulling out and withdraw the iron.

Repeat this process for all 4 inserts.

Next, use M2.5 nylon screws, nuts, and 12mm M-F stand-offs as shown to secure the Pi 4 in the base. Next to the header, just use an M2.5×6screw. Opposite the header, use a nut plus a 12mm M-F stand-off.

On the BrainCraft HAT, opposite the header, lightly begin screwing a nut onto two more M2.5×6 screws. Don't tighten them down yet.

Align the headers of the two boards and gently press down. Then tighten down the screws on the braincraft hat. You may need to apply a little force to the side of the nuts to stop them from just spinning. There may still be a slight height difference across the HAT but it should be minimal.

Finally, fit the top of the case over the hat and carefully slide it down. Gently bow the plastic on the headphone jack side so that it can pass over. The case will click shut. In the event that you need to disassemble it, you will have to bend the plastic out so that it can pass over the headphone jack the other direction.