The most basic "shooting bench" I saw was in one of the American shooting magazines several years ago..
The writer wanted something he could leave on site that was unobtrusive/low key that would escape the attention of idiots and vandals but still offer a solid rest to shoot off. IIRC he was either shooting on public land (looked like a semi desert area somewhere) or on land which had a lot of access by the public..
His solution was to build a "Y" shaped out of a 4" wooden post..Over all, I am guessing it would have been about 36" tall, with one arm of "Y" was longer than the other..
The bottom of the "Y" was then sunk into the ground at an angle such that when he sat down next to it, the tops of the two arms were the right height to rest a rifle on. The tops of the arms, the angle they were cut at and their height difference was such that the fore end rested could be rested on one arm, and the butt the other.
I don't recall, but i suspect he placed a small sand bags on each arm when it came time to shoot.
As the post were sank firmly in the ground and because he effectively "wrapped him self around it" in the sitting position, he said it gave very good results, but at the same time when not in use, it didn't draw attention like a full size bench would have...
I've never made one, but I like the idea as the result would be cheap and easy enough to build you could have more than one dotted around a property..
regards,
Peter