Inspired by the ever-excellent Mel, I made a handful of experimental bits of sandbags, some for 28mm, and others for 15mm. I based them on his Youtube video, which you should check out. They've turned out pretty nicely, and I mean to make some more. I'm going to make the 28mm ones a bit taller, too. I made these with air-drying clay, but future ones will probably have a top layer of Milliput so that the most visible bit looks better. If you look closely at these, you can see some cracks in the top layer of sandbags because the clay dried before I was done. If I apply a layer of sandbags made of Milliput over the top of that, it should look much nicer.