It could be 'sci-fi' concrete... like normal concrete but better
Maybe some sort of Kevlar based concrete with interlinked fibres that make the walls extra tough and resistant to impacts.
Above ground trenches/barriers make perfect sense. In Iraq/Afghanistan, they put hesco everywhere and all your trench is, is a path with a hesco either side... so there is perfect justification for your walls.
Also, what about them being flatpacked and hollow for easy transportation? They get dropped by grav-transport, folded open and filled with water/sand/'sci-fi bullet proof foam' for instant fortifications.
It is sci-fi so there are lots of opportunities to try
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