This simple stl file is provided as a quick way to confirm that your bed is level and properly zeroed all over, or that your autoleveling works as expected. If it is not the right size in any dimension, simply re-scale it in your slicer to suit.

I refer to this part in my book "Designing 3d Printers: Essential Knowledge". It was previously on Thingiverse by some other author, but has disappeared. If anyone knows who created it I will gladly add attribution, just leave the info in a comment.