[Duke]

What if someone drops something into the port? (Makes me think of those little games where you have to get the ball bearing through the maze.)

That was the only significant drawback I could think of, and the OP asked for "any sonic quality reason" not to use a top port, though I suppose there are things one could drop down the port that would affect sound quality. 

Once I designed a tall skinny custom speaker where the only dimension with enough length for the ports was the vertical plane.  I used one upfiring port and one downfiring port, both with flares, and lined up so that if you dropped something down the top port, it would come out the bottom port.   That's probably not practical in this case. 

[tortugaranger]

Well, after all that "top port or not" I've found a way to fit the port in the front baffle and get the drivers to line up on the center. Had to add a couple of additional slices to the boat hull to get the needed height but without changing the internal volume. These pics explain it better.

[JohnR]

Hi Morten. Cool! That seems to be a good solution. I'm assuming you've looked into tweeters with a smaller faceplate? Just wondering.

[tortugaranger]

Hi Morten. Cool! That seems to be a good solution. I'm assuming you've looked into tweeters with a smaller faceplate? Just wondering.

This is a project for someone who had already picked out and purchased their drivers so the cabinet had to somehow be made to fit. Fortunately it's working out. Plus once you start cutting and gluing the constraints build up quickly.

Latest design tweak is the addition of small embedded neodymium magnets under the baffle veneer in each corner to hold the grill in place. With grills removed there won't be any visible attachment points.