It looks like the pic shows the rear of the cab open, check the link below the pic it shows it wide open too, also it shows metal seats possibly meaning they would be exposed to the elements somewhat maybe?
Edit* the metal seats was an assumption on my part because i am building the RSO with pak which as an exposed drivers station with metal seats.
Oh and you should probably fix the title as that gun is the 3.7cm Flak43 not the 3.5cm Flak43. And where did you get all the extra 3.7cm ammo clips at? you didn’t get a whole buttload of LionRoar Flak43 3.7cm ammo with clip did you?
My educated guess is that it was open…crew comfort was not always the #1 priority for vehicle design…the armor was probably designed around an open cab to protect the crew from shell splinters or small calibre rifle fire from the front and sides…an open back would also allow the crew to serve the gun quickly…I found this and it does appear to show an open rear with perhaps a drape for keeping the chill out:
Italeri does do some nice armor subjects. Though you can probably get away with using the Tamiya Flakvierling 38 or the Tamiya/LionRoar Flak 43 in place of the kit issued one.