If I need 3.5 cuft for my 15 how big of a port do I need

[300spartan]

I’ll be running a sa v2 15d2 and I’ll be using a skar rp2000.1 at around 1500to 1700watts in my 300.
I was thinking of porting thru the 8” sub hole in the rear deck and invert my sub. So would that option make the box smaller as there is no sub displacement and port area? and how much external port area should be adequate?
I’ve never built a enclosure using my own blueprint but I have used many of other people designs

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[20 hz bass machine Premium Member]

Dakoustics has a good rule of thumb for this. He's a youtuber and does a good job going over port sizing.

 

[THATpurpleKUSH 5,000+ posts]

8 inch aeroport or 16-18sq in of area per cubic ft of box volume if using a slot port.