Just a box. Print in PLA with 0.16-0.25 layers. The walls are 0.8mm thick so it's only 2 layers wide. 

Print the inside with the flat side down and the outside standing up to avoid supports. 

On some printers, you may have to increase the outside xy scale up by 0.5mm to get a better fit. The model as-is printed perfectly on the Bambu, but was a bit tight on the Ender 3.