I don't know about "Recommending" it, but I use a bag of sand from Home Depot. 50 pound bag for a couple bucks goes a looooong way and fills many bags.
I use a small necked automotive (Oil, other fluids, etc.) funnel.
Put the funnel neck into the filler neck (may require some "force" and rotation to start), squeeze filler neck, add sand into funnel, a small amount (Soup ladle or so) at a time. I use a Swede Mauser cleaning rod and/or small flat head screw driver to assist/push/fill. Push it through sand in funnel, down neck and into bag. Pull back and forth to help sand enter, then push it around inside bag. Works for me and doesn't take too long.
Have also used a bag of lead bird shot (#7 or #8 I think it was). Wow, talk about heavy! That sucker didn't move! Alas, also expensive (About $25.00 a bag). Will stick with Home Depot sand.
Cheers,
Mark