I need to move a heavy-ish piece of concrete, a part of a garden sculpture, weighing about 300lbs, to about chest height. I'd like a hoist, but don't know the equipment I'd need to assemble an A-Frame-style hoist. What would I need to build a small, easy to move hoist for awkward sized rock and concrete formations? I need it to work in dirt, so wheels / casters are possibly not viable (like a mechanics crane has).
Rent or purchase an engine hoist. The definition of a small easy to move hoist, and pretty inexpensive, with some level of engineering built in (while you can still hurt yourself with improper use, at least you remove one area of hurting yourself with improper kludging of a lifting mechanism.)
They store pretty compactly and most can be easily taken apart for easier moving or storage in pieces. They have wheels, though small wheels will require some sort of base to run on or they are useless. Planks can provide that base on dirt, or you can not worry about moving it with a load on.
Never put yourself in a position where the thing you are lifting will crush you if it slips free (especially with rocks, which can be hard to get a reliable grip on.)