Eric, I agree with the other replies which indicated that a horizontal M22 isn't the best idea. If you absolutely have to use that space in the entertainment center, M2s properly positioned vertically would fit(and incidentally, as I've often commented before, a third vertical M2 would make a fine matching center speaker).
The article Peter linked was interesting. In reviewing Energy Veritas speakers Tom Norton made some comments on horizontal MTMs beginning on the bottom of page 2.
of the review. The comments and measurements page
illustrate the problem with the horizontal orientation. Note the suggestions that if the center could have been turned vertically the off-axis response would have been smoother and that the smaller 2.2 vertical would be a better center.