You can stretch grill cloth (or similar) over the vent, turning it towards Aperiodic. Peak vent output will drop a little. If one layer isn't enough, then try two, but the main thing is getting it tight. Once you have the right amount, make a frame to attach it to, and to the sub.
BTW, this improves transient response too, though how audible it will be I can't say.
Port noise is caused by the port area being too small. The velocity in the port should be kept below Mach .2 in order for port noise to be inaudible. Increase the area (diameter) of your port. You will need to increase its length to keep the tuning the same.
Well, you can stick a longer 6" tube in, but it'll probably need to be ~45" long to be effective, or you can double up or stick a larger one in, which will make it (them) even longer. Not good plans IMO, so this leaves either my suggestion, a combination of my suggestion and make it a little longer, or seal it up.