There is no predicting what "debris" you might encounter (if any), so no good answer to this question. The lighter the material in the skirt, the less it can stop. And nothing will stop everything. A piece of metal or a tire tread at 60 mph is going to bull through anything you can hang off the back (short of armor plate, anyway), or slip underneath it. There are basically three types: (1) hula skirt (basically a hanging broom), (2) rubber flap (solid rubber with multiple splits), (3) solid (one or two piece solid rubber).
There is also ample evidence that a skirt can actually cause things to kick up rather than preventing that. Especially if it is hung fairly low. Gravel or loose stone is prone to that, for example.
There are also other types of tow car shields that attach to the tow bar. Examples are the Roadmaster Guardian and Defender and Blue Ox Karguard.
https://jet.com/product/detail/9f6030da604e40bbac586f0c72a01be7
http://www.hitchsource.com/blue-ox-kargard-protective-shield-p-30324.html
I'm not trying to talk you out of getting one - just trying to set your expectations at a reasonable level.