I've got about 10 or so "banana kartons" which are the sturdiest boxes you can find around and I've just used them for my move.
They are about 50 x 35 x 20 cm so they are good for carrying small but heavy things (like books, etc.). They have a little hole at the bottom which won't bother you unless the content of the box is something that comes in small pieces. In that case, just put a sheet of heavy paper at the bottom.
They're yours for free if you come pick them up in Kreuzberg.
adam