Search:

rossi.45 one trick I did use before I brought a bag was I got an empty 3 ltr wine bladder. With the thing deflated it takes up on room and once you are in position just inflate it to desired...

I have / use one. Works well for sighting in but for me it's just another thing to hump around.
However it would work well as long as the ground is reasonably level. One you are on a slope I found...