..expressly forbidden...well ok..but that is precisely what i did at Basel airport on the direct instructions of the check in lady.
The box is cardboard. It is big enough for a standard size bike. I have a 64cm galaxy and I needed to remove my front wheel to get it in...had the box been another 3 " longer I would not have had to.
On th eoutward flight with Ryan air my bag was wrapped in a CTC poly bike bag. There were two other bikes on that flight niether were wrapped at all, just hand the bars turned in and pedals off. Both the unwrapped bikes arrived on the flight, but mine missed the flight and was unwrapped and totally searched to the extent that they even took out the seat post and looked inside th eframe. The bike arrived 5 hours later on the next flight after much frantic telephoning etc. The bike was damaged on arrival by Ryan air, the chain was broken in two places and the deraillieur bent....it looked to me like because mine was wrapped they picked on it to seach and in the process damaged the bike and made it miss the plane. So the polybag for ryanair in the event was not needed, in fact it was a positive hindrence.
I still have the easy jet box in the garage...if you need exact measurements let me know and I will venture out there with a tape measure.
I would add that I could not carry my own 'proper' bike box/bag as I was touring and had no idea where or when my return flight would be, so I could not leave a proper bag/box anywhere for the return flight etc.
If i were in anyway able to I would get a 'proper' bike box and use that when i flew (assuming I would return from the same airport.
BUT check out easyjets conditions...the lady at the checkin insisted that the only box they would allow was the one they provided...my advic ewould be to check with the airline well in advance as to the type of box/measurements etc that are acceptable and get it in an e mail to show the check in staff.
Also...I checked in VERY early at Basel, about 3 hours before the check in actually opened for my flight. This wa smore by accidient than design but in the event it was better because i avoided the nightmare queues I experienced at 4am with ryanair at stansted...which with a bike and all the gear is a nightmare. You would do well to have somebody with you who can help you lug the bike and gear around the airport if possible...at stansted nobody gave a toss about anything except themselves...it was chaos and dragging the gear through hoards of tired selfish chavs wasnt ideal.