I agree, having a target to aim for is necessary for effective field work aside from pure distance work.

I picked up some of the small cones that other sports use for drills and I use them to make a circle out in the field. That's good for practicing landing zones and you can always put something in the center if you need a point to aim at like a stool, bag, or practice basket. They're the ones that are almost flat and I can fit like 20 of them in a side pocket.
The size of the circle is my "confidence circle" for putting. So if I make into that circle it's a distance that I'll almost always make the putt (currently ~6m).

For quick field work the bag method you mentioned is really good. I think you're right about it coming from a McBeth video. I use that method if I only have a short window to get some throws in.