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...

Cut the sides: twice (390 x 25mm x 3mm) from plywood and twice (124mm x 25mm x 10mm) from whatever scrapwood you have left. Please make sure you calculate the dimensions correctly depending on the thickness of your plywood if you have differnt one: 130mm - 2*thickness of plywood = length of shorter side walls!

Now glue the side walls to the first base plate - using glue and clamps. Do not glue both covers - but only one side. The other will not be permantenly glued but fixed with two screws to be able to reopen the housing.

After the glue has dried - put the second cover on top, fix it with some clamps and carefully drill two holes on the shorter side walls though the (still loose) front cover and the side wall with a very small drill (3mm) or something to not break the walls. These two holes will later take the screws and make sure that the screws do not destroy the wooden sidewalls.

Now drill a last (bigger) hole - at least 15mm which will be used as a cable outlet from the housing. Start to drill with a smaller drill and then extend the hole further as the plywood is very thin and might break if you apply to much pressure now. On the side which will be top i added to hook to be able to hang the casing up.

Finally apply some paint in the color you prefer (I chose green) - after the paint has been dried you can add the ventilators and mount the complete housing.