Making a subwoofer from strange box. Does shape matter? What size driver?

I want to turn a structural box into a subwoofer to conceal the subwoofer visualy, but have it sound correct too.
https://drive.google.com/open?id=1QgQkg ... lfw3zMX8Gn
https://drive.google.com/open?id=1Gvlih ... tMic_Ws5Ed
https://drive.google.com/open?id=1ADAJE ... o37NErJRQ4
https://drive.google.com/open?id=1pUc2u ... axug9bN9Dn
The box is internally about 21" x 21" x 7" with one of the corners lopped off. It is made from 3/4" plywood. It fits in a corner and will have a large plant on it. The box was originally designed to hold a corner aquarium and is very strong and sturdy.
So I want to turn this into a down firing powered subwoofer and mate it too some nice Boston Acoustic speakers that are in that room.
I plan to increase the height of the feet all around, and add a bottom that will mount the down facing driver.
I estimate the volume to be : 21"x21"x7" - (7"x7"x7")/2 = 1.75'x1.75'x.58' - (.58'x.58'x.58')/2 = 1.8 - .2 (ft^3)= 1.6 ft^3
There is 15.5" between the internal baffles, so I could do up to a 15" driver.
So my questions are:
What size driver to use with this 1.6 ft^3 box?
Does the weird shape matter?
https://drive.google.com/open?id=1QgQkg ... lfw3zMX8Gn
https://drive.google.com/open?id=1Gvlih ... tMic_Ws5Ed
https://drive.google.com/open?id=1ADAJE ... o37NErJRQ4
https://drive.google.com/open?id=1pUc2u ... axug9bN9Dn
The box is internally about 21" x 21" x 7" with one of the corners lopped off. It is made from 3/4" plywood. It fits in a corner and will have a large plant on it. The box was originally designed to hold a corner aquarium and is very strong and sturdy.
So I want to turn this into a down firing powered subwoofer and mate it too some nice Boston Acoustic speakers that are in that room.
I plan to increase the height of the feet all around, and add a bottom that will mount the down facing driver.
I estimate the volume to be : 21"x21"x7" - (7"x7"x7")/2 = 1.75'x1.75'x.58' - (.58'x.58'x.58')/2 = 1.8 - .2 (ft^3)= 1.6 ft^3
There is 15.5" between the internal baffles, so I could do up to a 15" driver.
So my questions are:
What size driver to use with this 1.6 ft^3 box?
Does the weird shape matter?