Summary

Totally obsolete / useless / overkill Thingy for the REAL pedantic Thingiverse users.

UPDATE: Uploaded the Solidworks files.

Yes, you could do this with just a few pieces of wire.

1. make a cross
2. measure the distance from a leg to the center cross
3. measure a distance that should be equal
4. shift one side of the machine
5. repeat step 2, 3 and 4 until the distance is the same

Since you will need to do this only once better just use a wire :-) But for some people with extreme size MPCNCs this actually might help them square their machine.The tape measure can be used behind the yellow part.

So the whole idea is to get an equal legged triangle on one side. If all lengths of your machine are the same then you are square. You can check the other 3 sides. They should all be equally legged triangles.

(See hardcore PAINT picture)
AB, BC, CD or AD can be corrected. if your opposite sides are of equal length, than the X and Y dimension are perpendicular to each other.