i am from Belgium and know that the Tomorrowland festival is really great!
My advise would be to go to a decathlon store before going to the festival and get a small tent with them.
it costs next to nothing and they are quite descend! Get something to make your tent personalized so that you find it at night when you come back from the festival!
I would bring a small sleeping bag and mattress with you! Those are always easy if you do Couchsurfing or sleeping at somebodies house.