It is important to use a mounting rest to stable your rifle because it is the tool specially designed for this purpose. A mounting rest will keep your rifle more than any other object or tool. In a situation whereby a resting mount is not available due to one reason or the other, one can always substitute with the next available item. A shooter or hunter can improvise for a rest with their jacket, books, or their boots, as long as the rifle remains stable.
If not owning a mounting rest is what hinders you from using it, the good news is, you can rent one in a nearby shop where shooting equipment is sold. Yes, it’s that simple.