I made what I think is the ultimate bench rest. I took a scissors jack, welded a flat plate on the bottom, made a shooting bag holder out of wood for the top. You can adjust it fron 6" to 2'.
It will not budge, and will work for many sized people. The jack was 20 dollars at auto zone, cheaper from a junk yard.
On the way home from the range, if you have a flat, just jack up the truck with it.