A derivative to RyGuys Arduino Mega 2560 R3 Plate.
I removed the Bottom lid and merged both stl files into one with blender so I could print it in one piece.
It seems that the first version fit on mine but on some other it did not fit (I tried it with 5 arduino boards and 3 of them fit on it but the other two didnt).
I added a version 2 where i changed some parameters like the size of the pins to 90% and added 1mm to the length of the holding area. I hope this fixes most problems but please check the sizes with your arduino board before printing.
I used 10% infill and 0.4mm layer height. No support needed.
One edge bent up a little during printing but it was still ok.