Use the 50 shield city for freight and put the 20 shield city on a wonder. Since you can switch between a wonder and a spcaeship part with no penalty, you add the freight to the wonder until you have 300 shields in it and then switch it over to the Module for the last turn of production to finish it off. Four freight and 6 turns later you have a module with no wasted shields and two extra freight. If you use the 50 shield city for the module, in 6 turns you'll have a module, wasted 30 shields one extra freight and 20 shields towards a new freight. That's if my memory is correct and the number of shields for a module is right @ 320. Unfortunately I never even got a chance to load GOTM 16. But I'm trying to get a jump start on 17.