How about this for a proposal :
The kneeling position is defined as only 3 points of contact with the ground (2 feet & 1 knee).
The rear foot shall be upright and straight in line with the knee, sitting on a turned foot is not allowed.
A legal bean bag/seat may be used to support the knee, shin or foot area only, not the buttock or thighs.
The rifle will be supported only by the two hands with the rifle butt in the shoulder, no other part of the body can be used to support the rifle nor can the hands be supported on any other part of the body.
Please be aware that the hands cannot be dropped to allow the wrist joint, arm or any other part of the body to support the rifle. The leading hand should also be forward of the knee, to illustrate this the supporting hand should be dropped down vertically, if it contacts with any part of the knee/leg the supporting hand is deemed too far back & therefore not forward of the knee.
In addition, kneeling lanes should be marshalled by people who fully understand the rules, with some laminated photo cards of what is and isn't acceptable. After a season of everyone understanding what the rules are, it will become easier for everyone to buddy marshal.