Garolite = Amazing
Bought my sheet here, it's a 1/4" thick so you could go thinner but I wanted a stiff plate that stayed flat. Wherever you buy it make sure it's cellulose fiber based.
I've tested Nylon 6 and Bridge type plastic and both adhere perfectly, nearly zero warping, and what there was is most likely caused by me removing the part. I've used it at 0c, 50c, and 70c, the thermal limit of this sheet is 105c'ish. I did notice shrinkage causing elephant foot effect when printed while cold. 50c+ seems to be the spot. Removal when part is printed on cold or hot bed is equally difficult so it's personal preference. The shiney finish will come up when parts are removed but so long as you clean with acetone or high percentage 90+ isopropyl the same spot works just the same but with a matte finish on object. Video coming soon.