Gut feeling is (2) 4" Ports about 9" long.Let me number crunch and see what this looks like.
I think the above will get you a solid -3dB at 21hZ...-2 at 24 hZ and level at 28hZ.
This is strictly off the top of my head. I do prefer rear ports, 3 12th's the way from the top of the rear baffle. Space them symentrically side by side.
Now I will number crunch to see what this looks like on paper.