Most AC filters have a beehive-shaped spring attached to the top bolt that puts down pressure on the element. It isn't totally necessary, but it helps to keep the element from shifting around. It's not difficult to do a little creative tweaking to a hardware store spring to accomplish the same result without getting obsessive about the exact shape or tension.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!