width of hull panel second in from stern

[bigeasy1]

My boat(23 Tsunami) is 400 miles away so I can't measure the panel that is the second in from the outside.
Im installing another transducer and will be transom mounting it on that panel.Its for a speed and temperature sender. that sends the temp and depth reading from the down rigger weight to the display at the helm.
I will be attaching the transducer to a 12 inch piece of starboard mounted flush to the that panel.
If anyone can shoot me a measurement of that panel it would a big help.This way i can have it drilled and ready for my first trip in May without starting from scratch when i get to the boat.

Any help is appreciated You can see the item here.

www.fishhawkelectronics.com