Use the gauze from from standard sterile pads, they come in squares and expand out to about 5-6 inches, I soak them in a base color and then paint camo patterns on them depending on the terrain. They also have a second use, the layer of solid material can be used to make the bags on sandbags.
I use the sticky green floral putty that comes in rolls, cut a piece about two inches long, double it over and compress using your fingers until it looks like a sandbag.
Next, wrap the putty in the thin paper that comes with the gauge and shape to resemble the tied end of a sandbag. Then paint the whole thing in whatever color is used by the army vehicle you are placing sandbags on.
This method can be accelerated by an assembly line approach, start with the floral putty and cut all the basic shapes first and go on from that point to complete each sandbag.
The sandbags can be places in any configuration as the putty is flexible. I use all hobby/craft acyrlic paints and they go on with ease to color the sandbags.
Refer to photos of your armor/bunker, what- have- you to see how they were used. A note from Iraq; just as in Vietnam sandbags were placed on the floors of the Humvees to keep the concusion level down if mines were run over, the sandbags were double-layered, just as with the M-113s in 'Nam. My step-son was a section leader and his Humvee was equiped in this manner, lessons learned from 'Nam.
Good luck ! and hope this helped,
johncpo