Spend a couple extra bucks and get a custom box made, or find one that was fabricated specifically for your application. I've been doing fabrication for car audio/video on and off for about 5 years and you can trust me when I say that it makes a HUGE difference in the sound imaging to have one made specifically for your application by some one who knows what they're doing.
If you don't want to do that, and I understand if you don't want to spend the cash, I would highly recommend making sure that they are using at least 3/4" MDF or 3/4" Plexi. Also you will get the most bass out of the 12s if you fire them into the trunk lid or straight back into the rear of the trunk. If you fire them straight back you need some Dynamat Extreme so that you don't rattle your plates and trunk lid.