When I did plenty of testing on kneelers ...
I found if you use the front forearm along thigh, despite the fact that the hand is not supported ( it's easy to lock the wrist ... so the hand is basically supported through a stiff wrist ), then the front end of the stance ( front leg/arm/hand ) is pretty much as stable as sitting. You add to that stability by turning the front foot inwards ( to the right with respect to the heel ).
If I used a stiff souled boot and kneeled toes down with a high instep and buttock resting on back of heel ( so the stiff soul was taking my weight easily ) I found that that area was also quite stable and just as stable as using a bag in the instep/shin.
The area that isn't supported in kneeling and causes the whole position to be less stable than sitting ... is the whole right arm/right elbow/right shoulder/whole upper right quadrant. That now has no support for the bones. So that whole area can wobble around and cause wobble in the sight picture. In most people's sitting the right elbow rests on the top of the right thigh or on the inside of the right thigh ... or somewhere ... but it is supported by something so that whole upper right quadrant is more soild.
So no matter what you do regarding something under the instep ... little bag, big bag, roll etc ... and presuming you leave the rule such that the front forearm can be rested on the thigh with the elbow way back from the knee ... then that upper right quadrant will always be 'loose' and unsupported in kneelers and so the position won't be as stable as sitters.