Place a glass plate on the knife edge supports and center above the air holes on the top of the box.  The knife edge supports should be positioned at .227 L from each end of the glass plate.  The resonant frequency of the plate should be between 31 and 35 Hz.  Once the resonant frequency is found, turn up the amplifier until the glass plate breaks in half. 

NOTE:  When using new glass plate, it sometimes helps to scratch the center of the glass plate with some sandpaper to get it to break.  If all else fails, you can score the center of the plate with a glass cutter.  This will insure that the glass breaks before you damage your speakers by overdriving them.

To make the maximum resonant frequency more visible, you can aim a laser at the middle of the glass plate.  The reflection of the laser beam on the wall will look like a line that lengthens as resonance is reached.