I'll probably get flamed for this, oh well.
Maybe someone will have a better idea...
I like small well handling two-seater cars. That's what I've driven most of my life. So when I got the FRS, the first thing I did is put the back seat down.
I considered a seat delete kit. But they are expensive.
Just leaving the seat down leaves a flat, carpeted surface. Perfect. The problem with having the seat down is anything in the trunk is visible.
So I fabricated a simple panel to block the back like the seat does when it's up.
I made a prototype out of cardboard, painted black. It works fine.
The plan is to re-create it in plywood, covered in automotive carpet or something similar.
Ideally it will be two pieces with a hinge. so I can swing it up from the bottom to get into the trunk from the car.