Unfortunately there is a slight error in one of the former parts (SORRY!). The formers for part 3 should have a hole cut out in the middle just like the front and back cover of part 3.
@starbuck pointed out that the hole was missing. Major thanks for that.
I'm revising the kit on-the-fly as starbuck progresses and will upload a revision once he has finished (just in case he discovers more things to be changed).
This is how the parts are supposed to look, for anyone who is tackling the build now:
The small notch makes sure that the flap of part 3 can be glued to the corresponding surface as desired. Fortunately this mistake is not very grave since these formers are optional, just use them only if you feel it necessary. The graphic comes at 150 dpi and should be a quick replacement fix before the final revision is available.