Hi Chrisyt The shape of the boxes is probably not relevant. The fact that they are the same size is more important. Particlulary top to bottom. ie vertical linearity. Internal paterns from projectors are not brilliantly accurate and there for a guide more than any thing. Always best to use a source if you can. Draw a simple x hatch in paint if nothing else with an anmorphic circle to give you the 16x9.