huuuh, this was tough job.
i had to trick the program since holes have already been made, and i could not trim without an edge...lots of mirroring and re-combining, anyways, here is the result
had to seriously remodify the periscope top, though the interior looks much more "full" with addition of these ribs, maybe they are a bit too thick, but i made them bit larger so there will be some analogy among other parts in terms of dimensions.
what is done till now, with some transparency applied; note, when built, this will be "only" 52mm long (inluding the periscope casing), 42 without, wide 25mm and 31 mm high (all external dimensions):