Erwin, you might be pushed to find any suitable I fear; Resicast do some 6" shells but unless these are in the background, will be too small for your model:
Ammo set for 6inch howitzer (resicast.com)
I couldn’t get the picture to copy.
I just wonder if you could bend/curve something from these:
They’re made from white metal so just might work; they’re designed for the model farm community (who knew?) and are available from: