The guy that cut the plate for me talked me out of using 1/2” aluminum in favor of 3/8” and I’m glad he did. The only flex is in the pulpit itself and zero concerns about the integrity of the aluminum.
PM me your address and I'll trace it on a piece of paper and mail it to you. I still recommend you make a wood template out of cheap OSB to verify fit before cutting metal. I made a rough cut out of wood, clamped it to the pulpit and used a router from underneath to make it exact.
One issue I failed to mention is - by mounting it under the pulpit like this, the base of the motor was a bit too low. In the stored position, the head of the motor was resting directly on the gunnel making it difficult to unlock. The solution was I added a piece of 3/4" seafoam colored starboard on top of the aluminum that gave me the will wiggle room I needed, but if I did it again I'd use 1" starboard. The other benefit is covering the aluminum helped with the aesthetics.
Pic from the crows nest while looking for Cobia. I was going to hinge the rail but it sprung open enough to deploy so I just left it that way.