A forum to talk about Olympic style shooting, rifle or pistol, 10 meters to 50 meters, and whatever is in between. Hosted by Pilkguns.com
https://targettalk.org/
It looks like they don't specify how firmly you can stuff it, only that it has to soft, flexible material.7.4.7.12.5 Kneeling Roll
Only one cylindrically shaped roll is allowed for shooting in the kneeling position. The dimensions are a maximum of 25 cm long and 18cm in diameter. It must be made of soft and flexible material. Binding or other devices to shape the roll are not permitted.