Design optimized for printing with 

• 0.6 nozzle, 
• first layer height 0.2mm,
• other layers height 0.35mm.

Used the M2x8 screw with 4mm head.

The final print was rather tight, and the screws were not long enough to reach the nuts. I had to screw it first without the Pi board sandwiched between. Then, I tightened the screws to press-fit the nuts and the screw saddle. After unscrewing it, inserting the Pi board, and screwing it back in, everything was fine. The screws were secured well over the entire height of the nuts.