An easier way to check how close your X and Y axes are to square is just to drill 3 holes, the same diameter, to make a (should be) right angled triangle. i.e. Holes at (0,0) , (X,0) , (0,Y). You can then measure the 3 lengths and use that to either calculate the angle, or draw a diagram in a cad program and measure it. There are less errors in this method then cutting a shape, in addition to it requiring less material - could even just use the bed if it's MDF.