just the volume of the box ( size). if your lining it.
i dont think it will help but if it does i would like to know.

are you sure its the port and not the dirvers bottoming out?

Top port noise seems to be a re occuring problem for some.

I had that sound with my 8 hwk but it was the drivers bottoming out in my case.

Is the sound NEW?  
Could it be the top flexing or loosening the screws, slightly?
Do you have a leak around one of the drivers? That could cause the drivers to bottom out or if it's leaking on one side the diaphram will be loaded unevenly and at high power could cause it to mis-align and rub. There are so many things that can cause "klopping" sounds.

Your foam could be just the fix for the right problem or no effect for others. If it was "my" noise, I wouldn't want to try to fix it until I know what's causing it.

try playing some really low sine waves, like 15-20hz so that you can focus on the noise.
try to take the top off and play some sines/music. if the noise persists with the top off it means it's not the port.
i'd do these things before lining

The foam did help, but the drivers were brand new and now that they are breaking in, the klopping is better. I did tweak the eq some so I cant really say for sure what helped. I'm using a Wharfdale pro 15 band eq, but i have a 31 band Alesis laying around and i'm gonna give that a try.