Thanks to Brian Enigma who posted the OpenSCAD File to his Mario Questionbox (http://netninja.com/fun/mariobox/). I was able to modify it and add a thin filling for the questionmark.
Size is 10x10x10cm
Put a LED light into it and close the lamp with the top cover.
I used a 9V battery block with a handfull of LEDs.
1) Print the box
2) Fetch coffee (Printed on the Ultimaker with 0.2mm layer thickness in 5.30h)
3) Print the cover
4) Put some light source in it (e.g. LEDs, fireflies)
5) Close up
If you use PLA, you may need to print it on a heated bed to avoid warpage.