easiest thing to do is aeros .. lets say 3 13inch aeros tuned to 45hz.. plug one its tuned to 36.. plug two its tuned to 28hz.. or course that was just an example not exact math.. what do you want to do? compete and have a daily or just have different ports to change for the heell of it
you could build slot ports that are interchanagle depending on box depth.. might have to sacfrice more.. aeros or pvc ports are eaier to interchange plus easier to plug unless you build a special plug for the slots
You could build ports out of PVC (correct me if I'm wrong, I don't do ported boxes.. this is just an idea) and you can get those threaded pvc plugs/caps, they use them on septic systems..if you know what I'm talking about.. when you want it sealed or tuned lower just plug one, or two or three etc..
they are about 12-15 bucks depending on where you get them.. partsexpress sell them for a reasonable price. and yeah you add about an inch to the port lenght if you use a calculator that does not have aeros in it.. i use bass box pro 6 but it has both single flared aeros and both ends flared.. calculates it all.. great proggy