Sorry,can't give you any links to the reference,..but if you cut the morser karl ribbed plate and the part on the bottom of the square it will give you a better feeling for the dimension of the box,..but be shure that the box only meets the fuselage tubes where the crosstube enters the fusselages(if you deside not to cut square holes in the fuselages but only round holes for the cross tube)) .
So in other words,.the morser karl plates/decks are wider than the square stock on both bottom an top!
This will also mean the holes in the cross tube have to be further removed from the square stock to be positioned exacty where they should behind the canopy and octagon hole.
The square stock will be covered by styrene that follows the curves of the fuselages on the front,..maybe back aswell(not sure),.so this will cover up the voids which are between the square stock top and bottom and the fuselages.
I'll see if i can take measurements from my build ,.but these will be my measurements so others or you may not agree.
I remember that my thickness of the cross tube was slightly different than most other builders used,.. i believe the only one agreeing with my dimensions or at least not ridiculing it was monsieur tox,.who i take as expert ,so i stayed with my measurement.
You could also cut square holes in the fuselage tubes(more accurate to the filming miniature) so you will not have the voids ,.more difficult for me so i just cut a round for the cross tube.