My first experiment was with 2" perforated angle steel. Easy to work with but not strong enough.
I need to go with your idea of the square tubing.

A climbing jumar from some of my climbing gear was an easy cheap device for controlling the lines.

The left image is from 3 weeks ago in WY.
The image on the right is the same guy this week with an elk quarter above Creede.

Notice on the right image the boom has bent considerably but still usable.

[Linked Image]
[Linked Image]