Build a shelf/cover

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I have a car (Ford Transit Connect Wagon)
It comes in two wheelbases
The short one has two rows of seats
The long one has three (I have this one)
Until a few months ago, Ford used to offer a "security shade" for the long wheelbase version which was some sort of rigid shelf that covered the space behind the second row of seats (the third row would either have to be folded or removed)
Now they only offer one for the short wheel base.
Now, I kind of want to have one but because I can't find it, I'd like to build one.
It would be made or two pieces hinged together (for easier installation in the vehicle), each piece about 4 ft wide and 2 ft long.
I can do plywood but that is too heavy for the strength I want (I want to be able to put something like 10 lbs on top of it without it bending too much)
The question is: should I even consider fiberglass or carbon fiber?
Anyone has any experience with these materials?
How hard is it to make planks out of fiberglass/carbon fiber "cloth" + resin?
How hard is it to make "wavy" planks (for added strength)?
How hard is it to make a "sandwich" out of two smooth planks and one "wavy" plank

>yes, I'm still trying to find the original Ford part; I'm checking CL, Amazon and eBay every week or so