There are a lot of interesting guesses about this vehicles
In my opinion the vehicle is based on Stug 4 which would make conversion in reality also a little bit easier (basic frame from StuK ) . The Sockel of the Flak looks like Behelfslafette which would make more sense for mobilising.
About driver compartment:
Sorry to say but from technical side Dragons version would work, but all this selfmade flat roofs with hatches would not work in reality because this roof would be at the height of drivers waist. You like to drive with arms blocked from hatch?
So for reality you need (Stug)interior for your Cabrio.
And please forget all ideas to put this heavy gun simply on steel plates. The torque from recoil would destroy it immediatly.
wiseacre mode off.
Regards from Vienna
Visit our modelling show "Go Modelling" in Military Museum in Vienna at 10/11th march 2012