I wanted to make my Eighth Army and DAK anti-tank units have some sandbag protection and authenticity, so I made my own.
I rolled up long lines of Milliput clay in the same way we all rolled up Plasticine a long time ago. Then the lines were chopped up with a blunt plastic edge of something, I forget what, and left to dry. Took about 25 minutes and was quite boring but I think the results were ok. Like ballast, sandbags are one of those things that look okay out of scale. In reality these bags would be too heavy to lift!