While working on the easy (low detail) version I was able to make it "fold-flat". Very important to gamers who need to store their huge collection of terrain bits or transport them safely to the next gaming location.
For the easy version I compressed all the details into the texture. That way I was able to squash the parts flat. I also made the barrel just four sided.
To assemble you squeeze the barrel into the right shape and then insert the back end piece. For the gun carriage again you squeeze and tuck in the back flap (inspired by the wedge) to keep it in the right form.
I will go for the detailed version for my ship(s) but I am very happy that I also found this solution!
Chris