Replacement for metal shed steel panel

[eqa]

I have an 8x4 Outsunny metal shed that was delivered with some incorrectly measured parts. Have received a full refund with the kit left in my possession. Basically wall panels about 3 inches wide are missing- they're to be screwed into the base frame. As is the case with many metal sheds, the panel presumably holds some of the weight of the shed but not too much considering its size. What material could I use to replace it? Polycarbonate roofing sheets come to mind.

 

[foxhole]

Any manual pics? Metal may be required for strength.

 

[eqa]

Don't have the best of pics- took this one just put together (not screwed) for the manufacturer as metal sheds do really feel like they're going to break in the wind until fully assembled.

As you can see, there is panel on the left. If it was correctly measured and cut, it would extend to the last hole on the side of the base frame (that hold is just about where that very visible hole on the top of the base frame is) where the corner panel (which I have) would start. 3-4 inch unmeasured gap.

What do you think? What material can I use? I can rearrange panels if structure will be more stable if the gap is at centre of the wall rather than near the end. I can also replace roof (something plastic/vinyl/polycarbonate) if that can lower the load compared to metal.