I have two identical hft kneeling bags one has the proper polly balls filling the other is 50 50 with those polly packing wave shaped things and polly balls. Can't see any difference between them except the wavy things are free.
I do have sand bags at home but they are not as good as the real thing and far to heavy to carry around.
Don't use the packing wave things that are made of pva and dissolve in water test them first.