Need help with box

[Wrestler157]

I'm new to the car audio and just recently started redoing my old 88 Toyota pickup and need to build a sub box for a focal p25fs. The truck is a single cab with an uneven floor in the rear so hard to make a box for a beginner like me. Can anyone help me out? I will post some pictures of the area and measurements. Also I would like it behind the passenger seat and try to keep it low as possible.

Should I build a shelf to level out the floor then build the box on top or can I build it another way? The bottom measurement can be about 7.5 inches and top about 4 inches. From floor to top can be about 17 inches. I can always move the passenger seat up a little as no one ever really rides in it.  The sub recommended a sealed box. Thanks for any help