Hi Sven
Since I've not used my account for a longtime, the ancient e-mail was no more active so had to change my profile data. As regard your acute observation this is upon your desired diameter, for my project there were in fact residual voids (LEGO doesn't produce many odd stud pieces). Since, as you know, I'm not a purist, I was imaging to put a heavy black cardboard (or a self made sheet of black plastic dish) between the frontal door and the boiler body.
regards
Sergio