For those with rifles with butt-hooks (particularly Jeza and his new AWP) here is a ridiculously effective, simple and cheap rear-rest idea, that I would recommend:
It’s 2 bits of pvc-covered metal garden cane and some tape.
I do ALL my prone AWP shooting using these since I find the bean-bag/sock idea too low. Under ideal conditions (or in a tunnel range) I regularly attain 0.3MOA with this rest. In the field I expect 0.5MOA.
I carry 2 versions at all times in my smock pocket, a 5” and a 9” to suit ground conditions/bipod height/shooting angle.
The tape padding at the tips allows you to ‘lock’ the rifle vertically even if this rear rest is canted/uneven due to ground.
In use; make sure the rest is not vertical, but leaning back slightly into the butt-hook. Use your butt-hook hand to grasp both rest and stock. On hard/dry/very soft ground you may need to use this grasp to prevent the rest from splaying.
[Jeza, With the AWP, lean firmly into the front bipod. Be conscious of the surface you are on, if hard (dry mud etc), I recommend you put a jacket/similar between the bipod and surface for consistency of MPI. Then simultaneously lean down firmly on this rear rest]
Hope this may be of help!