In other words, identical.What it looks likes behind the seats.
Always a pleasure to see other well informed enthusiast speak.Look in the back of a 370Z for the answer. Same silver bar, same security shade. The new Z might have a tiny bit more space at the back due to the different placement of the taillights but it's ~98% the same.
Compared to your Mustang, the trunk is much shorter, but the opening is far wider and taller due to the hatch. Remember that this is a 2-seat car (it isn't a 4-seat anywhere in the world, unlike some of the classic Zs), so it's short behind the seats. And there is a considerable structural brace hidden that runs between the shock towers. Furthermore, being designed up front for an IRS rather than a solid axle that has to allow for considerable space to move in it's travel up and down, the floor is very low (unlike the Mustang, which still has it's carryover floor from the previous sold axle chassis).
The silver brace in the 370Z and new Z that you've seen isn't structural at all... zero... it just prevents your "stuff" from moving up against the back of your seat. See below - note the simple attachment points for that. You wouldn't for example, want to run a safety harness around it.
Here's that the 370Z actually looks like under all the plastic and carpet. It's very well-architected and very strong structurally.