Old guy with soldering iron
Join Date: May 2006
Location: Portland, Oregon!
They are the centers of the drilling locations. The size of the ports isn't critical, by the way, wavelengths where they are used are long. Just don't go out to beyond where the driver gaskets will be. And for best vertical pattern, I wouldn't let the height get any taller than in the drawing, want the centers as close as reasonable to the center of the waveguide. Too narrow height might possibly restrict air flow (though not likely).
That drawing is for 0.5' plywood side panels for minimum size and weight Be careful to make sure the back is well braced, maybe more than shown in the plans. My original ones were actually done with 3/4" ply, and seem quite dead, though.