At first, it would seem wrong to make this square, but since Tracy used air horns inside the box, that seems to make it OK to me.  The box itself is just a big plenum, or reservoir of air, so if it's big enough, I'm guessing the shape doesn't matter that much. 

 

Rusty, You got it in one. The box shape isn't too critical if the volume is big enough. A flat side toward the air horns isn't a bad thing either, rather a good one. Try to make the height of the flat side about 2X the diameter of the air horns for good flow. I know you are hurting for space, so just make it as big as you can. Past the air filter the biggest you can fit in is best.

Bill Jepson