Hi, I’m super late to this thread… but maybe useful to someone in the future:
I use the shipping flats from Amazon
For my first year i was also deconstructing and hoarding amazon boxes…it’s good cardboard
But they take up space, and take time both to break down and to cut to size.
The 12x20 Flats are perfect close in thickness to the boards i use, and 11g steel (When i’m sending the finals off to Plasma.)
They come in a neat stack. No dunnage (they are dunnage) just shrink-wrap and a strap. They line up perfect on the bed, and are consistent thickness
One downside in shipping the corners of the top & bottom 3 can be pretty smushed. There are two sellers i’ve tried from the first had crisper edges… i’ll dig up both links and share shortly