I call these yard bags as they hold a square yard of materials, and one of those things that just look great filled up sitting on a flatbed trailer.
However while I have found these around, most were made from canvas with the seams on the outside with the stitching showing, but over the years I have learnt that when making scale items its a compromise between the real version and what looks right when scaled down, and to me this stitching just looked too big.
So after playing around with various materials, I found reinforced plastic sheet not only looked the best, but sat really nicely when full too.
These are thermally welded, not sown as I did try sewing some, but couldn't get the stitches small enough to look right, so in the end put the seams on the inside and went for overall look when sitting and full.
As you can see from the pictures the straps had no problem holding one full of stones, so will be fine to fill up and load and unloaded off your truck at home or at club meets.