In my other project i wanted to install an air ventilation to facilitate air circulation in my greenhouse. As is still had some standard pc fans which are quite energy efficient (1.8W) and the greenhouse runs on solar power i decided to go for these. Air ventilators offered for greenhouses are able to transport more air (my three together manage 210 m³/h) but use a lot more energy. But this decision of course required to build a houseing as no such thing could be found (wood is fine for me). What you see here is the results (see the sketchup file) for details
Cut the base plates (390mm x 130mm) (twice for front and back) using a circular saw or anything else which gives you good results
Mark the middle of the hole on both base plates according to the drawing. Important is to consider that the side walls will take some space away - so you have to measure them first and from there evenly distribute the mid points of each circle on the midline of the base plate. Draw the circles (55mm radius for my Emermax fans) around the marked mid points using a compasses.
Saw out the inner circles using a jigsaw. Drill a hole in the middle of the cirles first - then cut the holes out. If you have some utility with your jigsaw to saw exact cirles this is very helpful - mine does not have such a thing - so my circles are not perfect unluckily...