Buy some 12 X 12 tarps.
Cut them in half, roll & staple (roofing nail) the edges to a 12' - 2 X 4. (or a 2 X 2 )
End up being about 5' wide.
They roll up & store easy enough & are easy on & off.
Have some weight to help keep them on & give you a solid connection for bungees down to the pallets on the ground.
If I did that now I might lay pallets on top of the stack , then put the tarps on. Better air circ.