I would suggest contacting Warbases and seeing of they can do something for you in lasercut MDF. The towers and turrets are both quite boxy so should work quite well in the medium. You can detail the surface with a mixture of laser etching and appliqué bits glued on.
Old Crow do barrels and some nice resin turrets as well or you can bodge something from bits of brass and or plastic tubing.
Magnets can hold the turrets in place while allowing them to rotate and be removed.
I was thinking about getting them to do some turrets for me in the new year, maybe even the bunkers to mount them on as well.