The front panel of the computer had a plastic part that was holding a small board with some red and green leds. As soon as I had stripped the board and wires from the plastic part I thought "Shelf brackets!" So I glued a piece of plastic backed fabric to cover the holes.
Then I cut the shelves from thick cardboard. The options now are either painting the whole thing or covering the shelves with something and painting the rest.
The size of the shelf unit is about 8 cm x 10 cm with 2 cm deep shelves. I haven't decided where I want to put it, but I made it already, because the part was so suitable for this particular purpose.