Does anybody know if camo netting in 1/35 scale is available, or a method of scratch building some? That would really look much better than the gun being out in the open as it is now.

There are some commercial camo nets available. Verlinden has some as do other. However, you can do it yourself. Two products come to mind: cheescloth or simple 2 by 2 medical pads. These are an open weave cotton product, folded over several times. Any drug store or large department store will have these in the first aid section.
Open one up and dip it in thinned acrylic paint. When dry, drape it over the gun as you see fit.

if i'll find it i will post the url for u

I am going to put some sandbags round the front of the gun

here u are sir. hope it helps

A natural cover might suit bettter ... a ditch or wall or ruins!... Even hidden in heavy foilage. It seems more likely that they have just set up and are quite relaxed. If they were gonna fill sand bags, they would probably dig-in a bit as well

A diorama will look far more natural simply by eliminating parallel lines. It would probably be better if the roads in your scene were not parallel to the frame. Even skewing them ever so slightly would result in a more appealing effect.