Sub Box for bench seat?

[TP121]

Can someone explain to me how to put a sub underneath a rear bench seat?
I have seen guys talk about "downward firing" subs- does this mean I build the enclosure, put the sub facing the bottom of the boat, and bolt the whole box to the bottom of the seat?? Or do I put it in the compartment, on the "floor"?
I have a bench seat with 3 individual compartments- I was thinking of sticking a 10 inch woofer in 1 compartment just to pick up some low end..but I'm not sure where to start.
Where can I find out how much enclosure volume I will need for a given woofer?
Any help would be great!

[Outnumbered]

Check out this thread:
http://www.***boat.com/forums/showthread.php?t=105472