http://alshamswiremesh.com/hesco.html
This type of item filled with large gravel, without the fabric liner, was being used to reinforce the faces of cuts (e.g. in streambeds, to resist erosion) back in the 80's or before. I recently saw the modern version up close and personal at the Carlisle, PA US Military History Institute (Army War College), which has numerous outside military displays of several time periods. This one was a modern display.
I think that the defenders of Sielce might be able to scrounge up some chain link fence to build the wire cages out of, lined with scraps of salvaged carpet, perhaps? A tripod swing arm with a block and tackle could be built to maneuver the filled gabions into place. Any guesses as to how many man hours each hexside of these would take to place--build cages, fill with chunks of rubble, wire lid on, hook up to crane, swing into place, secure with tie-wires.
It's like filling and placing giant sandbags, but you have to make the sandbags themselves.