This is an open topped test cube - you can make the cube the size you want (stl is 15x15x40 with walls 0.48mm thick) and it can have marks every few millimeters (or not) to allow you to see where changes are made. The openSCAD source is included (it is very simple) as is a sample test cube in gcode (which will print centered around 0,0 for a delta).

I hope that people find this useful.