I bought this led power supply from ebay. It fits perfect under my Prusa and is strong enough to also power my heated bed. It is called "DC 12V 20A 240W Switch Switching Power Supply Driver For LED Strip 220V AC" and the size is: 19.8cm x 10.9cm x 5cm. I got the switch and the power connector from an old pc powersupply.(Bought them now too for another Prusa, they seem to be very standard e.g. RSonline 440-6979 and 488-191) The cables are not in the front anymore like in the picture because this made it a bit hard to assemble.
Assembly:
Print the cover,for me 1 perimeter with 60% rectilineair infill and 0.4mm layerheight worked allright.
-Remove the two small supports for the hook and the cable hole
-Fit the 3 m3 nuts in the nuttraps. (if the nuttraps are to thight put a screw through the hole, put the nut on, pull the nut into the nuttrap, if needed heat the nut with a soldergun while pulling)
-put any tiewraps in that you want to use later to fix the powersupply to something
-Solder the wires to the switch,
-Put the switch in the cover,
-Solder the wires to the powerconnector
-Screw all cables to the powersupply
-Slide the cables in the hole beneath the power connector
-Screw the power connector to the cover
-Put the cover on by first fitting the hook around the screw that holds the pcb
-Press the other side in.
-Put in the two screws
-Drill a hole in the power supply through the remaining hole in the powerconnector
-Put a tie wrap through the hole to fix the connector to the powersupply.

On the pictures is the former version of the cover. I tried to solve the last small problems in the 3d model but I have not printed this newest version yet because it was allready good enough for my printer. If you have an nice idea how to enable inserting tie wraps afterwards please let me know.
Good luck