here maybe some ideas:
This one sounds pretty good, they still have few rooms available from March. If I would have found out about it earlier, I might have tried this out.
Registration as a "basic" member doesn't cost you anything, and you will get many offers for rooms! "Premium" membership costs about 20 euros for a month - "satisfaction guaranteed".
The downside with appartager: as basic member you can only read & send messages from and to premium members.
But they really have a lot of offers, and many of them are premiums, so you can send them a message. And you will get many offers to your own search advertisement as well!
I almost went for one room through them, but then I found a very nice flat just for myself! Good luck with your housing search, and see you all in less than a month in Brussels!