More theoritical ideas...
Make the Hull big enough to actaully include the fixed 'turreted' section too.
The TURRET which would control the guns elevation and arc (note most of the SPG's do have a horizontal arc, just very small, Jadgtiger 10 degrees either side of centre)
The TURRET would have the gun on it and would be INSIDE the HULL with just the gun showing, that way all you see when the turret turns is the gun moving, as the hull covers the rest up...
This theory MIGHT work, no garantees