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I'm not too lazy to tell you that it's a stupid idea.
For one, as far as I know, it's illegal to cut up the metal of the door... It weakens the door and makes the car unroadworthy.
Also, 6x9's don't give the best sound... Why do you want 6x9's in your doors? Which doors? Front or back? If you want alot of midbass from your doors, work out how to fit 8" ROUND speakers and do it properly.
The main reason is 6" or 6.5" in the doors and a 10"-15" sub in the boot will sound 100 times better than 6x9's in doors and will be a hell of alot easier.