Standing waves will begin to occur at the frequency whose wavelength is equal to the longest dimension of your box. So in your case, 22.5", which is a frequency of 603 Hz. Obviously well above your crossover point, so you don't have to worry about them!

Standing waves only become a problem when the speaker is playing frequencies which are close to or above this threshold. So they will only be a problem with woofers running up to high frequencies or woofer enclosures that are absolutely humongous.

Adam