I found a nice power supply unit in an old computer that was lying in the cellar...
By "nice" I mean : adequate size, in metal (and quite strong), possible to transform, with already-made fixations holes (!)
The tool I used : a metal saw, a drill, a good file (a rounded one), few screwdrivers, a soldering iron, hot glue.
The technical piece that were added : a 4 port usb powered hub, a two port usb holder (from the same old computer), 3 RCA port (from an old dvd player - still working after that I desoldered it), an ethernet port (from a dead router), 1 loudspeaker (at the top).
The RCA port, usb's from the usb holder, the ethernet port are linked to the r-pi using home-made "extenders" so that no raspberry pi was modified in the build (I can unplug the board whenever I want).
I have also made a "very short plug" for the micro-usb so that I would not waste space.
Everything in the box is removable (because holded with screws).
The power come to the board from a usb cable (that is not unplugable - yet). The box is not finished : the hub I have is dead so I will buy the same on ebay (I broke it because it was on the table where I used my soldering iron and a very tiny bobble of iron jumped on the board of the hub !). I also need to manage to add a power witch and a 1.5" screen I already have. The loudspeaker is not used yet because I need an amplifier.
Here you have 9 pics, the last 2 are in HD.