Another alternative method
Yes, use a separate bin bag or large barrier bag. Fill with chosen stuffing, I actually do use old clothes, bubble wrap and polystyrene, BUT, I stuff the bin bag to size required, get small plastic bag full of sand (couple handfuls) and place that inside on the 'bottom' section.
Then tape the bin bag to shape (seal ends then couple wraps round middle to get tube shape) then stuff THAT into sandbag.
Reason is, I get loads of packaging material, most is none recyclables, plus the bubble wrap and polystyrene are water proof.
They keep the shape and sand gives bit of weight. They sit ok, can stack ok, if out of shape just plump them up like a pillow.
I've done hundreds like this, not a problem.
Another tip is if using the large airbags (I get mine free off a mate) take them off the roll but keep flat, then cut down the middle to make two halves (not across centre, give wrong shape)
Then tape seal the seam shut with brown packaging tape to make your saussage shape for stuffing.
You can adjust the weight with the small sand bag inside if want bit heavier.