My Opinion -
Form should ALWAYS follow function. If something is built correctly, and functions as planned, the form of that thing is beautiful as is.
What does this mean for your project? Ditch the back seat.
1. It's unnecessary weight
2. It won't do anything, other than "hide" the fact that you're driving an EV, which is pointless, given your propensity to "show it off" anyway.
3. It takes away from valuable cargo area that might otherwise be utilized in a better way.
In addition - Something like Lauan (very thin overlay panel, can be gotten cheaply from wood stores and
Home Improvement
Depots...
) might save you a few lbs for the cargo area cover. Using thin polyacrylic (plexi) might keep you from ever actually having to open it up, save for modifications and pointing out specific details, and Lexan is even lighter than Plexi is, and doesn't scratch as easily.
Scratches for the poly would happen when moving the carpet back and forth... over time, just the rubbing motion would dull the Plexi, while it might not dull the Lexan panel.
These are my 1/50 of a dollar for you, my attempted contribution to your project.
PS - did you get the pics of the springs?