I once saw a bag like this that you could solidify with a suction pump. I will be experimenting. Emptying surplus polystyrene balls within a larger bag was reminiscent of using a photographic dark bag - those balls will get everywhere if given a sniff of their freedom. A 50/50 by volume mix with split lentils works well for me, balancing weight and firmness whilst leaving plenty polystyrene to to-up in use. A real find for any photographer seeking a convenient solution to camera shake on location. And if you want more and are prepared to find a willing sewing machinist, try out Army pattern DPM camouflage polyester fabric. Fully waterproof. Oxford 300D. (100cm x 148cm).