The TIE Fighter (and also the Cylon Raider) has no real reference point to determine the scale. An X-Wing has the droid which can be used for measuring because we know how big the droid is. The only area which could be used is the cockpit interior. However we only know that one pilot fits inside who is able to look out of the front window. There are no fixed numbers of the size of the pilot's chair or how far it is away from the window. So the only way to determine the scale is guesstimating the dimensions as good as possible.
I like the idea of a plain mockup. There are many designers who build a white model which only has the linework on it first to see if the parts fit together and to check the final shape. I did the same when I was working on the Landram. You could mess around with it, correct proportions on-the-fly, cut it open, scan it and trace the changes to improve your template. I'm really looking forward to seeing your progress!