Charging Station

Well after a lot of searching the shops I decided to make my own extension boxes (charging stations). I need to be able to charge 12 batteries simultaneously every night for the next 2 months. (There are 18 batteries, maybe 20)

And instead of charging 1 or 2 at a time, I decided it was easier at the end of each day to simply charge 12 at a time. This means I can put the batteries on charge and do other stuff, as it's likely I'll need 10-12 batteries per day. (In fact, I have no idea how many batteries I'll need but i'm barely getting 500 shots from 2 batteries now, and I'll be shooting 5-10k images per day). Worst case scenario is that i'll need all 20 batteries, but it takes 2 hours to charge, so I might get a couple of goes at charging each night.