building an enclosure too small

I am almost done with the frame for my glass box. Question though, I am putting 3 15" subs in a box, all chambered off. The left and right sub is fine, but the chamber for the middle sub will be smaller then the other 2. I measured the middle space, and it's 16"D, startes off at the sub at 15"W but then as it heads towards the back of the box, ends to 9"W and 21"H. Now being that is smaller then recommended, would it be a smart idea to stuff the middle chamber with polyfill?. If so, how much?

Another question, like I said, the box will be chambered. But it's very hard to chamber with so many curves in my box. Does each chamber have to be airtight?. Or can there be a 1-2" gap near the top of the box?

i would recomend not chambering the box. if you have 1 sub that has different airspace than the other 2 it is going to sound like a$$. just put some internal bracing and cut some good size holes in you already existing chamber walls.  this will make all 3 subs have an equal amount of airspace resulting in better sound.  and polyfill isn't a necessity.  in my opinion it doesn't make a big difference. none of the boxes i have ever built have polyfill.  find a website that has professional competitors and look at the boxes that have plexi on them, do you see any polyfill?? didn't think so