Art,
You are correct sir. Aluminum is easy to work with but inside the  lazzerette 
it would be subjected to high humidity and possible corrosion because  the 
bolts used to mount the motor mounts and gudgeons are stainless steel. Also,  
the introduction of any salt into the equation simply speeds up the corrosion  
problem and you would have to add a great deal of thickness to get the same  
strength as stainless.