If you wish to experiment with building concentric ports as detailed on the long ports page, here is the calculator.

Input the size of the small tube and the wall thicknesses and it does the rest. Choose a starting size that is large enough to keep air velocity low but small enough to keep port resonance above the sub's working range. Play around with different wall sizes to arrive at the size you want visible.

If you are unable to source a pipe of a particular size, you can make it out of 2 layers of PVC pipe. The software has an extra page which will help you to work out how much to cut out.