If the door was easily installed then I am sure you are using the right one.
I had tried to install the standard battery door with the extended battery when I first got it just to see if it was possible; I found that with even quite a bit of pressure I couldn’t get it to work. I have heard of people actually managing to get it to fit, but most of them noticed a “rainbow” effect on the screen and even loss of capacitive control. If you have to force the door on, then you’ve got the wrong one.
You can either place an order through Motorola by phone, to get just the door or you can now buy it on Amazon. The correct item skew is: SJHN0449A. However the door all by itself is rather expensive and you might want to consider just buying an extra battery and door.
Here is the link for Amazon:
Amazon.com: OEM Motorola Extended Battery Cover Door for Motorola DROID X, SJHN0449A: Cell Phones & Accessories
Or you can buy the package which includes both the BH6X and the proper door:
Amazon.com: Motorola Droid X MB810 OEM Extended 1800 mAh Battery BH6X with OEM Door SJHN0449A: Electronics
There is really no valid reason to use the standard door with the BH6X as a vast majority of all the cases on the market will accept the 1 millimeter of added thickness.