So your needing to protect already in place earth bags that have been tamped into the barbed wire stabilizers from the elements, Yes?
Plastic can work for this, you need to angle the sides out so there is air space between the completed bag work and the sides of the plastic.
This support can be simple but it will be needed to prevent snow from collapsing the cover tight against the completed work I would think.
Some rocks along the bottom of the sides will help hold it in place in winds and a fairly steep angle to the sides will help snow to slide off instead of simply adding weight to the plastic cover.
Something looking like a pup tent
should work quite well and allow air flow too.