200x200mm Bed Level Test / Border

Only 0.2mm thick, this uses the whole of your bed to check if it's level, sneakily missing the corners, if you have clips holding your glass down.
It also make a great priming border around your prints and shows your instantly whether your filament is coming out or not (when I print something small I can't always see whether there is a problem...).

Put it in your start up g-code!