"Can I just make it a semi-cube? 24x24x25 so that the inside dimesions are still the same but the outer box can house the front cover? I think this would also extend the port by an inch, so will that ruin everything?"
if you keep the outer dimensions the same as the cube, but push the front baffle in as in your picture so that you can make a flush mount grill, all that will happen is that the internal volume will be reduced. that will have the effect of increasing the tune by a bit.
here is a rough idea. no eq applied to either. green line is about the martycube. black line is the marty cube with the front baffle inset by 2.5" to allow for a flush mount grill.
it raises the tuning frequency a hair and decreases low end sensitivity by another 1-2db.
on any of the other marty's it is negligible, but on the martycube, which is already very tight, it actually has a bit of a noticeable difference.
it will still work of course, just a smidgen differently.