I've seen ports installed between the pylons. IF (that's a big if) I were ever to do it myself that is where I'd put them.
Now, let me say this, I personally have never installed ports on any of the sixteens I've ever had. I don't think cutting a hole in the deck can do anything but degrade the structural integrity of the boat. I read this topic reposted time after time (in different ways) and cringe at the thought of one more boat dying before its time. Need air in your hulls? Pull the drain plugs out after every sail and jack the tongue of the trailer way up in the air. Moisture in a hull for even 24 hours isn't going to seep into the fiberglass that quickly, now over the course of a Winter sure it will. A couple of soft spots here and there (in the deck) can be repaired WITHOUT installing ports. Doing a leak test and sealing up some places where water is coming in is time better spent.
Thanks for letting me vent.
There is always the cooler caddy (mounts in front of the mast) and soft sided tramp bag to store goodies in while pleasure sailing.