The volume of a cylinder is (pi)r^2(h), so:
(pi).5^2(12) = 9.42 cubic inches, or .005 cubic feet per brace. That's 0.04 cubic feet for all 8 braces, which is virtually negligible.
As for ports, do whatever is easiest. I've seen great results with both. You don't need (or want) a lot of port area with those. About 9-10 square inches per cubic foot is plenty.