I understand all of this. Im just looking for some max dimensions to work with, not that i will be building it that big. I would just like to know the largest possible box that would be able to slide in the trunk and toward the seats.
That may still vary on whether you are willing to build a custom shape for maximum enclosure space or just keep it simple with a rectangle. Either way its on you to chose the difficulty level and get busy with a measuring tape.
For the rectangular box consider the following measuring suggestions:
1- measure the narrowest width of the opening when you drop down the rear seats.
2- Crawl through the lowered seats and measure the width between the 2 trunk arms.
3- Chose a max width between both measurements thatll allow you to slide it in from the front then wiggle it into place from the trunk opening and allow for trunk closure without obstruction.
4- With the trunk closed, measure from the spare lid to about inch or so below the lowest point under the rear deck to again ensure no obstruction.
5- Lastly, guestimate box depth that'll allow you to slide in the box & move it as needed. My guess is around 10 inches.
This may not be a specific response but it'll give you the specific numbers you need without blindly trusting someones numbers that may be inaccurate.