What I use is a combination of a cheap bench rear bag filled with poly beads and a homemade rectangular rear bag filled with poly beads. You adjust the rear bag by five steps.
1. Set the bipod as low as possible to clear brush and grass and give a reasonable neck position.
2. Determine how many bags you need under the stock to get the approximate alignment.
3. Rotate the rectangular bag to give a pretty good adjustment.
4. Slide the bags forward or rearward so the angle of the stock gives you more precise adjustment.
5. For the last tiny bit of adjustment squeeze the rear bag with your left hand; however, that hand and arm has to be perfectly still so it can't be much of a squeeze.
I have a loop of twine on each bag and I have a clip on my pack to clip the bags to the pack. I also keep everything I need to make a shot in one bag that is also attached to my back pack. When I take my pack off, I unclip the rear bags and the shooting bag and lay them next to the rifle so I have everything near by. I place my earmuffs over the scope so I can't forget to put them on.
I will say this as tactfully as I ever say anything, less than one percent of the long range hunters can shoot in a hurry and kill anything. If you get in a hurry then just plan on missing and scaring the animal.