i had the same exact problem. what i did was i ripped the entire top part off... and that was hell.
after that, u have more room, but no wrench will b able to fit in it because of the design..so wat i did was i used a flat philips head and put it on top of the clamp that was attached to the gasket. i then used a hammer and hit it hard so that i got a gap between the gasket and clamp. then i juss that all the way around and i got the stupid clamp off. it was very tiring, but rewarding. lol
if you wanted to keep the airbox, then im not sure how to remove it, but if u wanted to break it, then i suggest u do wat i did and rip the top part off.