In a STURDY box, gently tied down so nothing is loose, and with NOTHING heavy in the box unless it is SOLIDLY tied down.
Its ugly what a winder loose in your box can do to a wing.
I use a long lawyers file box with foam rubber glued inside and cutouts to hold the plane parts.
Now,its convenient to have everything together so some like to have their flight log, repair kit (you do take a repair kit with you flying, don't you?), winder, extra rubber, etc so nothing is left back at your school or team room on the day of the tournament. I think its better to develop good personal discipline and keep a seperate flight box for the heavy stuff, but... So if you have to have one box, make sure NOTHING is loose.
I've heard and seen many horror stories about plane boxes being sat on and destroyed, so consider this when I say sturdy. Some teams use 1/4 inch plywood boxes with reinforced corners. On the other hand, I've been using the same cardboard box for my SO demo planes for 10 years now around hundreds (if not thousands) of kids with no damage (to the box, no comment on the parent who got too close to my plane!). Your plane, your choice.