I recently went through this same search. I finally decided on floor spill absorbent such as mechanics use in a garage. It has the density/weight needed and it's relatively cheap.
https://smile.amazon.com/gp/product/B00GP57QVU/

I use cat litter. It makes a dusty mess when filling so do it outside and don't get it wet. Works well for me.

G

Split pea lentils. No dust, no odour.

Hello, thank you all for your response's to my enquiry.

Tried the cat litter, bag weighed 5.2kg and it was very firm.

Our nearest ASDA (ex Walmart) supermarket has 1.75kg bags of dried split lentils, or dried red beans, or dried chick peas.

I liked the feel of the chick peas because they appeared to me to be a little more forgiving than the other two.

So, cat litter out chick peas in and my bench rest bag now weighs 4.1kg.

My air rifles now sit in the vee of the bag rather than on the top of the bag, and appears to me to be a more stable support.

Update 2: Chickpeas out and now have polystyrene foam beads in.

Yes, the weight has come done dramatically that has made the bag much easier to move about and carry.

Have found gun rest to be stable such that I have achieved pellet upon pellet shots down the range.

I have been pulling my air rifles down slightly into the rest because the beads do settle.

After using the rest today recon it would now benefit from the addition of another half pint of beads.

The beads I have are the flame-retardant beads approved for furniture fill such as bean-bag seats.

One cubic foot appears to be the smallest quantity that is available to purchase.

I found it enough for filling the gun rest bag and stock rest bag that goes with it, leaving some for topping up if required.

Cat litter is £1.99 for a big bag. It is lighter than any stone type material, quite easy to fill the bags with a folded piece of paper or card.
I have just filled 5 sets of front and rear bags for the shop on the range here, with cat litter