well, finally on friday I suddently remembered this wooden frame in my garage
(it's part of an IKEA shelf - we have two of those and a cabinet inbetween - which we use now as changing table).
I cut the "ladder" in the right length and inserted the pieces I cut off at the sides, so it became a frame:
If you have such a handy tool (whatever that is called in english) the rest is easy...
Tada!!! A fun piece for baby boys room. And so educational! We expect him to know the continents at his first birthday! :)