I make mig covers out of 3/4" scrap plywood I get at construction sites. Since the dimensions are 14" by 23" (8 frame) there is plenty of scrap around that the job forman let's me have for free. It will last forever if you rub in waterproof outdoor silicone caulk with your finger into the edges and then paint with two coats of outdoor semi-gloss latex paint. Use a couple of bricks as hold downs and it will keep it from bowing.
Do you paint both top and bottom sides?