Two thoughts I have before you commit to this. One figure out how to add 150 to 200 pounds to this area and make sure your happy with the performance of your boat. It may not make much of a difference in your boat but it would be great to know. 200 pounds at the very back of any boat can really change a lot of things. Maybe tie your buddy to the transom and take him for a ride
Next build a mock up version of the box at its largest size and make sure everything works right not to mention can you service powertrain parts etc would be great to know that what youve designed wont hinder a repair or make a minor repaiir a huge repair.
I added a swim deck to the back of my smaller boat years ago. I made some rough estimates as to what would and would not work but never actually made a mock up of it. I did get the angle right but I could have made it 4 to 5 inches deeper making it much more comfortable to sit on in the water as its just big enough for a fat guy like me to sit on.