Hi SSpanzer
You could go for the log trenchs like in Band of brothers
Got a question for ya- is the flak looking after and airfield, railway complex or a defence postion or is it going to be mobile.
If it's to protect an airfield or the like a log or wooden trench will llok good they also had the 20 mm up in towers or in cement bunkers.
If it's mobile they would just take it of what ever was pulling it and use it.
As for stuff around it would find that the only things around it would've been helmets, ammo cartridges spent shells(if in combat) bino's and maybe gas canisters.
Ron