I honestly think that you're OK, and certainly this is true with the overall cabinet dimensions. You'll find that if you go one direction - towards overdamped - the bottom cutoff frequency strats to rise rapidly after a certain point. But until you reach that point, response doesn't hurt much by being a bit overdamped. And going the other direction, you'll reach a point where peaking starts to rise fast. Before that, you aren't going to have a bad response curve and can barely tell the difference and then suddenly, there's nothing you can do to remove the peak. So those two points - one towards the overdamped and the other towards underdamped - are what I'm talking about when I say "in tolerance." The window between these two events is pretty wide for PiAlign'ed cabinets. It's a very conservative alignment.

So that 1/4" is almost nothing, so as long as its braced sufficiently, you're in business. And I'll check the response curve for the port and double-check my assumption on the cabinet volume too.

I'll let you know by tomorrow afternoon.