I have revisited this and simplified it significantly, reducing the file size. I added a base footprint for the printer bed, which will remove any need for the brim in printing. Thanks for your patience
This is a 28mm remix of Marko's file.
I up-sized it to 28mm, created the support beams for the roof, which added some height to allow based 28mm figures to stand upright, added texture to the wood parts with Meshmixer, and mirrored the sand bags on the left side to the right side to lengthen the sandbag coverage.
To finish I primed, painted with cheap acrylic paint, and dry brushed. I added medical gauze, coloured with craft paint, to represent a poorly placed camo net.
please note the support beams go along the side, as in the pic. I glued with with epoxy; CA won't work because the fit isn't tight.
BTW, that is a 3D printed 28mm British 17 pounder ATG in the pic; it is a big gun: you can fit a ten man section in the bunker on 25mm bases.