We got a new HVAC unit two months ago at work for one of the offices. It had a big box with some excellent flat cardboard, so I nabbed that. I keep an eye out for it all the time. Boxes that photocopy paper comes in iare very good. The top is a good flat sheet and the sides, while not too tall are good and straight generally with no creases. Anything that has a flat surface area bigger than a 10x15 sheet is what I look out for. Then I use the hold down pins or strong magnets to ensure flatness.
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