I had the defenderworx locking fuel door installed on my SIM - you can find the install instructions here
http://www.defenderworx.com/images//...uctionsint.pdf
Note that the installed clip on the locking housing causes the door to stick up a bit so the left side of the door will not be flush - its a slightly uneven but still looks great.
I took one of the keys and put it on a lanyard placing this in the driver door storage to grab easily and the other on my key ring.
I recommend NOT giving the key to the gas station attendant but doing it yourself so they don't scratch the door or force the key.
Vigilance comes at a price of doing the above to keep your ride safe from door/gas thieves and uncaring gas jockeys.