sand sifted through a fine sieve then left out to dry on a piece of cardboard , basement floor stir it once a day for a few days vey dry , But i use small kids blue jean legs from the 2nd hand resale shop make several sizes , nylon zip ties on each end and your done
Depending on the fit and finish of your bag material, some filling products may work better than others. Some prefer something malleable, others need weight and firmness. I've tended to use some 6mm plastic BBs from my airsoft days that have held up pretty well, i can shape the bag(s), but they're rather light. Nice when carrying them around but you can scoot them on occasion. I've gotten used to that and like being able to adjust that way sometimes.
Started using a soft bag only recently. Rice for now but I want to change it ... too gritty especially with a high power BR scope the image jumps too much.
The original source of leather sandbag rifle and pistol rests made in USA since 1938. Offering top grain leather shooting bags and rests for hunting, benchrest competition and long range shooting. Also manufacturing aluminum shooting rests, leather accessories, trap/skeet bags and rifle slings.