Ron, You made a valuable point when you mention being able to back up. Most of the drag harrows (the simplest DIY variety especially) do not permit backing up and in fact have rather severe manuvering restrictions. In tight quarters or where manuvering is a must, a rigid form that permits backing up is a wonderful feature.
I have only used primitive improvised drags that restrict your turning radius as well as elliminate backing up. Of course out in a pasture that is not a problem but I have to stay away from any need for intricate manuvers (or get off and disconnect, manuver, and then reconnect, a procedure to be avoided.
Maybe I can come up with a mod to my King Kutter el cheapo rake to use it as a harrow.
Pat