Put your Arduino in an Altoids case. This has blocks that hold the PCB and can be "melted" with a soldering iron or similar to keep the PCB in place. Also, there are 2mm holes that can be drilled out to your preference to use screws to hold the Arduino in place. There is 3 mm of space under the PCB, ample room for wires and leads underneath.
Access for USB programming, power, or other connections will necessitate holes through the Altoids tin, but I leave that as an exercise for you, gentle maker.
Printed with 3 perimeters and 50% infill. There is not a whole lot of infill, given the thickness of the part, but a little in the mounting pads. Let it fully cool for maximum flatness before prying it from the printing bed.