Test box - Wall Thickness

I need a box for a Raspberry Pi & Relay Board but need to figure what thickness the walls should be and what size vent holes I can use without adding support material. This box has 0.5,1.0,1.5 & 2.0mm walls. It works well with 1mm walls which uses 2 layers and the best types of vent holes are 3mm squares. So next I need to design the real thing. This does illustrate the limitations with 3D printing - especially the RepRap.