Hello!
Well, this is something that i can imagine very well, because the world before the modeling in my life was made of plasticine figures, houses and vehicles. I think that another way to make real-looking sand bags is to give them just a pillow-shaped form - without shaping the top of the bag, aslo ithe form of the bags may be uneven if you like. Then you can add to the Miliput bags the texture using some cloth with sharp pattern - maybe silk or nylon. Just press the cloth a little to make a print marks on the sandbag. I don't know the features of Miliput substance, but for me it looks like plasticine.