I need dimensions for exact size for a 15” JBL 2225

http://www.lansingheritage.org/images/jbl/specs/pro-comp/2225/page01.jpg

http://www.pispeakers.com/JBL_2226.pdf

I need only to have a signal port centred above the driver similar to the JBL 4645C.

I’ve checked out costs for 18mm MDF with cost approximated at £45.00 pounds, excellent.

Now all I need to know please, is how low can I get this to go to within technical tolerances.