Corncob (like they sell to reloaders for cleaning cases in a tumbler). Cheap, and lighter than sand (and less messy).
I once took a client out varmint hunting, using a vehicle and some 'shooting rest bags'. He was a small statured Italian fellow who, inadvertantly, pulled the gun closer to him and AD'ed into the top 'shooting rest bag'. Rained sand for several seconds. The vehicle, although sandy, was unharmed.Shooting rest bags