Ok I made plans, tell me if this is good

Im going to make a 3 Cu/Ft box for 2 12" Polk Mo Mo Subs. I did some research and found out that 30 Hz would b about the best for rap music. I came up with 3 cu/ft cuz each sub needs 1.5 in a ported app. Im going to make both subs firing up with the port in the middle. Its going to be 12.5 Inches long and the diamater of the port is going to be 4" . Can someone tell me if this plan sounds all good?