Milliput (two part epoxy putty). Mix it up and roll it out into 'sausages' the thickness you want for the sandbags. Then cut the 'sausage' into the lengths you want for the sandbags. If I am placing the sandbags onto something, e.g. a Sherman tank, I will use the putty as soon as I have cut it up, because it is softer (fits onto the surface easily) and stickier (adheres better to the surface). It is possible to leave the cut up sandbags for a few minutes (e.g. 5 minutes at room temperature) they will be firmer and this is what I do if I am making a stack of sacks. The advantages of this system are: The sandbags do not look starched & put together with a set square; They can be drapped over other things; Minor damage (bullet holes/tears etc) can be easily modeled before it sets & detail can be added (e.g. make a small embossing tool & press a logo onto each bag, such as 'Melle' or D.A.K).
I hope this is of some help.