I would line your whole shutter with a sheet of polythene if your mixing and pouring it your self mate , it'll stop the water in the concrete soaking down through the base making the concrete much easier to work with , will also help to get a tidy finish.Regarding cement make sure you mix it plenty strong enough , dog piss, constant pressure washing and frost will find any weakness and it will start to crumble , unless your planning to tile it ?