Behind that middle seat I am looking at about 6.5 inches maybe a hair under, but if someone sits in it im sure it would be less. I think that pioneer box is the thing that will fit.

 

The subwoofer box that you are looking for is not available in Canada.

"Pioneer of Canada didn't arrange a supplier to build these boxes
like the USA has for Pioneer to sell."

What this means is that I would need to build you a box based around this boxes dimensions, not a big deal at all if this is what you are after. It takes a half a day to get a basic upholstered box completed.