Cheapest panel would be 1/8" hardboard/Masonite. Currently both Menards and Home Depot local to me have them for $5.98 each.
It's $8 a panel here. Everything in Maine is typically more because of the distance. This is looking like my first choice for now. I know it might get wavy, so be it.
What is a good fastener for hardboard?



Sorry for the rant.
