ports for enclosure

You better use a port calculator if you want to change anything regarding ports.  If you use 2 instead of the 1 port, the length is not halved.  It is far more complicated than that.  The length would actually increase by a longshot for each port.  Use  this calculator and enter specs that will show a 4 X 9 port.  Then increase the port number to 2, and you will see the increase in length.  There are other port calculators as well. Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Go to Home Depot, buy a length of 4" PVC tube, cut it to size & paint it whatever color you want & slap it in a box w/ some Liquid Nails.  It's that simple.  Make sure to completely round and smooth the edge of the port that is inside the box to prevent port noise.  Make sure the outer edge is cut level so it sits flush in the box.