My guess is that they were built using whatever materials were at hand - compacted earth, sandbags, fieldstone, or anything else that wouldn't blow away.
It's not Axis, but the Wikipedia article on aircraft revetments has two good pictures of RAF blast pens on Malta. One is made from stone blocks, the other from old fuel cans filled with sand.
https://en.wikipedia.org/wiki/Revetm...45._CM3241.jpg
https://en.wikipedia.org/wiki/Revetm...45._CM3237.jpg
Anyhow, textures are up to the designer, and probably should be quite simple.