Sorry bout the clarification needed... so if you cover the current port holes and if it's 2.4ft3 net... your sub installed will knock it down to around 2.3ft3... add a 4"round at 11" long... and it should end up at 2.22ft3 tuned to 33hz according to this calculator....
This box is pretty big... and should go low well... that power you've got is not all that much ... anyways... this is how I'd do it.
I am going to build a new box with my dimensions I figured I will have 1.76 cft net with a 3" diam X 6.5 inch long tube it will be tuned to 35 hrz. Thx for all the help