What look are you going for?
If you frequantly haul stuff in the back an aluminium or sheet metal panel would work.
If it's cosmetic only black vinyl(or what ever color you want)with a small amount of padding on the back side could be mounted to a 1/8"sheet of plywood( a replacement door panel would work.Used to reskin a house door.It's thin like that.)
If you can find some factory ones(goodluck)that would be my choice.I know they reproduce them but they are about $400 bones.
I got mine from a guy in B.C. for $125.00.A good deal I think!
Here's mine
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71 blazer,350SBC,approx.375HP,700R4,factory GM TPI.Dual electric fans,33x12.5x15 ATR on stock suspension.
Petrolia,Ontario,Canada but working in Port Huron,MI.
See ALL my Blazer pic's
HERE