need box ideas

[JohnCouch]

i have 2 12" subs in a ported box that sits on my back seat in my 99 chevy silverado extended cab and am really wanting to put the speakers in a box under the rear seat but not sure of anywhere that sells that type of enclosure so looking to build it myself and could use some help on the design on the box i saw an enclosre on crutchfield but it was for 10's

thanks in advance for any ideas