T2K Fortification

[micromachine]

Sea cans would be good with sandbags on the outside and the inside, not to mention some chain link to act as " heat killer", and you have a viable keep. You would need to incorporate grenade sumps and ammo storage, and some way to prevent water infiltration.
The only real problem is how do you move them, cut holes, and make them into fortifications without power tools, lift trucks or boom trucks? To be sure, they could be nudged about by tracked vehicles, but we are really talking about precision emplacement, and nudges (burning precious fuel) are only broad strokes.
Sandbags, sharpened stakes, shovels, axes, saws and picks with be the bastion makers of this era, coupled with copious amounts of manual labour.