You are just trying to add mass, so if you can get the bags in it should be fine. Considering that you’re not trying to fill up 100% of the space (or shouldn’t), bags would work. That said, you can “massage” them in but may not be able to do the same on the way out even with your thread idea, so only do this if they go in really easily.
Additionally, the manufacturer is pretty clear on their suggestion of how to do this (from the Arro manual), so not sure you need to be more careful than what they suggest doing:
“Note: Always remember to apply the plastic adhesive stopper to seal the cavity. To prevent damage, the speakers, should never be transported with mass inside. The cavity must be emptied prior to transport by using a shop vac and sealed thereafter using a sturdy tape to avoid any loose sand from migrating. Totem recommends the use of either: Kiln dried sand; Silica sand or lead shots.”
Erm, if you fill up part of a speaker cabinet you change its internal volume, thereby you will change its sound, possibly quite significantly. One effect may be to reduce the depth of its bass output, another may be to change internal resonance and standing waves, potentially changing the overall character of the part of the spectrum handled by the bass/mid driver. So unless you are merely completing a design intent, perhaps omitted by the manufacturer for transport weight reasons (and by so doing bringing it to the design spec), it would seem a very odd thing to do.