I still use the bonnet of my Landcruiser which is not standing up but leaning over too much. I think now my Landcruiser's suspension has been fixed and lifted it might be better. Maybe using a box of some sort would now work??? Will give it a try right now ... yes it feels much better now and would probably work well as an adhoc standing rest.

For a better longer term solution I would try sinking three or four long posts into the ground, and either drill holes in them with pins or bolts, or use clamps of some sort to move the "bench top" rest to the right height for the individual person. Would work for me as it would be on my own farm.