I wasn't giving advise, Quitty, just pointing out the fact that guitar cab sizes are not designed according to acoustic principles. They are based, some more closely than others, on the size Jim Marshall arbitrarily chose when Eric Clapton and Jimi Hendrix asked him to build 4x12 speaker cabinets for them back in the 1960s.
Those same 4x12 cabs are what we have heard for over 40 years, and what we are still accustomed to hearing, when we hear rock and metal with distorted electric guitar.
...And none of them are square. Not without extensive internal damping, anyway.
Saying that guitar speakers are not FRFR speakers, while perfectly true, does not mean we can ignore what makes them work right. We're aiming for a different effect, not ignoring the effects altogether.
Standing waves are bad, period. For any speaker.