Mike,
I just purchased and installed an MC2 chrome billet locking fuel door. Check this site out for it.
http://www.t2hummer.com/T2Hummer/fueldoor.HTM
I think this is the best quality fuel door on the market. It's also pretty hot looking. Installation was pretty easy also. I bought mine on ebay for $169.00 shipping included. This door weighs a ton, solid construction with a heavy duty lock.
2003 Yellow H2, Lux Series, Air Suspension, Front Brush Guard, Rear Tail Lamp Guards, MC2 Chrome Billet Locking Fuel Door