From an aerodynamics standpoint, an external round flared port is your best choice; this is also why it works so well for those who compete in SPL.

The obvious issue is that flared ports require a little more work. If you want to flush mount them, you need to get the router out. You should also stabilize the port in some way. Additionally, it is challenging to work with a long, round pvc port, whereas a slot port can easily be tied into the enclosure and bent at will.

Again, there are great advantages to a round flared port, but a slot port is usually easier to integrate, particularly for tuning low.

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-Neil-