thank you for going above and beyond!!! this REALLY helpsDo you have a power rear window? Mine does so that was one of the reasons I couldn't use the Kicker behind the seat. I think it's also a bit too deep. Unfortunately I didn't try it out to know for sure.
Here are some measurements I just took to try and show the available space behind the seat.
It seems 18" is the max width that would work with a power rear window without cutting away at the adjacent plastic trim to the left.
Roughly 12 1/2" between the upper bulkhead and the top of the cab vent.
Depth I'm not 100% on, but for reference the face of my Amp is mounted 3 1/2" from the back wall.
Top of my amp is about 1/4" from touching the seatback with it latched. So approx. 3.5" max depth at the very top of the flat area.
About an inch of space between the lowest point on the face of amp and seatback. I'd say about 5.5" max depth where the wall starts to angle towards the floor.
I think the easiest option for behind the rearseat is using a really slim, all-in-one unit like the Kicker Hideaway. I've tried a couple variations of those types and was not a fan, several others on here seem to like them though.
Otherwise, for a prefabbed enclosure I think the following dimensions are probably max.
Upper Depth: 3.5"
Lower Depth: 5.5"
Something I didn't really consider until now, but I think a spacer for the catch that's mounted on the back wall and some longer legs for the seatback would be a relatively simple way to increase available depth and accommodate a deeper enclosure. I'm not sure how much could be gained though; I'd think 1/4" or 1/2" would be relative safe?