Actually it's not eficient because of slab volume and how is easy to find iron and carry.

My proposal is:
-use 12 girders for the frame
-remove 5 side of the block
-add 8 slabs for cover 4 girders with pillars.

So will it can have and be:
- ~17 slabs
-only 1 covered side
-less time for build
-same time for grind because is concrete
-same damage resistance
-buildadabe with 2 travels 400L
-fortify the idea to be a floor or roof and not armor
-be original without similar mod