For this first attempt, Marc used gaffa tape as a hinge, but he says he'd use a proper metal hinge if he did it again(!) In terms of a closing mechanism (i.e. keeping it closed), Marc says to put the phone in a silicone/TPU case. Hmm....
Anyway, thought you'd all be interested! This is a somewhat drastic solution, but does mean that the device's cell can be swapped very quickly. Why the heck did Nokia not put in something like this in the original design?
PS. He also says that, when swapping the batteries, if he's quick then the E7 keeps time and date properly.