FSM magazine had a multipage step by step instructions on making sandbags some time ago. Sorry but I have no idea of the issue or dates.
On rhe wire...soaking cotton thread in half water and half PVA (white glue , woodworkers glue, etc) and wait until it is dry enough to bend to shape works. This only works with cotton not nylon thread and some colouring will be needed to disguise the shiny result.
Finally barbed wire on battlefields was often placed directly off the roll so it has sort of a spiral / spring shape to it.
Good luck ,
Pete.