Suggestions for 'filling the port' of bass reflex

I used expandable rubber test plugs available from the plumbing department in any hardware store to plug ports in my PSB Stratus Goldi speakers. They happened to be a perfect fit and made an airtight seal. They come in 1-1/2, 2, 3, 4" sizes and if they don't fit for you you can wrap an undersized plug with strips cut from bicycle inner tubes to get close, then turn the wing nuts to expand the plugs to seal the ports. This is a cheap and easy solution unless your ports are on the front and you don't use covers. Talk about low WAF!

I had good results cutting soda straws to the length of the port and then simply packing it full. This reduces turbulence and overload without changing the tuning of the port. Using fabric, wool etc. impede the air movement is not only a gamble, worst case it can damage the speaker.

I've used filter wool refills from a tropical fish pet shop. You can buy large bags of it for a few dollars.

Try an Acoustic System pro resonator ( copper) for internal use. It brings a lot of good things !!!!!!!!!

The bonded dacron that I suggested will turn some of the acoustic energy in the port to thermal energy, and thereby damp it's resonance. Done correctly(not crushed or tightly packed), it won't impede the woofer's motion. It certainly will not damage the system in any way.