I'm not understanding the need for 'cookasheet'. I work on a tile and just put the tile in the oven, or put the tile in an aluminum tin and cover it, or put a piece of parchment on a tile and put the piece on the parchment (if I don't want a shiny side on the clay) and bake. The tile is very flay, the clay is easy to remove from the tile, and the tile dissipates the heat so you don't have 'hot spots' in the baking area of the oven. And, they're cheap, easy to clean, with lots of sizes and shapes available in lots of places. The clay items either don't stick to the tile, or if they do, just slide a blade between the item and the tile, and it pops right off.
Hugs, Boni

in Hot Wonderful Yuma, Arizona