Interesting. I did some research, and it appears that mounting a Flak gun on a truck this size was a field improvisation. A Flakvierling would have been quite a heavy weapon for a relatively light truck like an Opel Blitz - a Flak 38 seems to have been more common (see below for an example). Often the side panels were removed altogether (as in the example below). The Maultier seems to have been fitted with a Flak gun much more frequently.
It also appears that Rubicon's Opel is a relatively early 4x2 version (as shown in the second photo below). All 3 sides of the cargo bed could indeed be lowered on this version. The later version had lower sides with wooden slats along the top, and a canopy which extended much further down.
Anyway, it's all good - very much looking forward to buying a few of these kits!