For my
Willow Feeder, I found a contractor nearby that regularly does building projects using the type of metal roofing on the Willow Bank. They had a pallet full of dozens of "slip sheets", the top and bottom sheets from when they get pallets of the roofing shipped to them. As they have cosmetic damage from the straps holding the pallet together, they set them aside and sell them for 1/3 the price you would normally pay for a new one at a big box home store. If you end up wanting to focus your time on other aspects of the build before next winter, there might be a contractor or two like this around Missoula...
Go for the camo effect for bonus scrapper cool points: