[EricH]

Mecanums do not have to be in a square pattern (wheelbase), though it may help turning to have that.

The rollers may form either an X or an O; this may be judged based on contact pattern or on looking at the top view (which is the opposite of the contact pattern). The good news is that if you don't like one pattern, change the wheels end-for-end and you get the other one in maybe 15-20 minutes of work. Just make sure that you've got either an X or an O, or your sliding will be off...