Couple of box questions

[Another Device 10+ year member]

I'm trying to design a ported box, and here's what I've come up with. I need 2.3 cubes for the sub, and the displacement for the sub is .2 ft3. The port is circular, 4in by 13.5 in, which makes its displacement ~.1 ft3. So the internal dimensions I need would be 16.75x15x18, which gives me 2.6 cubes. Did I mess anything up in my calculations?

Also, If I made a fiberglass box, how much support would I need for the fiberglass, as in, how much space should I allow between each piece of the skeleton.

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