"Do you think the skis make a big difference in tracking? Other designs I've seen either use tracks pre-molded into the plastic bottom, or add a thin aluminum fin bolted to the bottom."
Actually, this design does not track very well on sidehills. I built it especially for groomed snowmachine-type trails, which usually don't have the terrain that a backcoutry hiking trail may have. I've thought about adding a fin or two, but am not sure about that yet.
"I'm not sure what your cut-up skis weigh, but it might be over two pounds."
Biggest disadvantage is the weight. Nordic skis would reduce it, but would require buying kid's skis. Another option would be attaching a sheet of high-desity plastic to the bottom, or even a few strips of the plastic that is put on dog-sled runners. I like the last idea. May order some of that.
"I've been meaning to make a pulk from an old plastic toboggan, but my kids won't give it up!"
Hehehe. I have a old plastic toboggan pulk as well. I use it for hauling heavy weight to a cabin (you know, beer weight). The plastic has a lot of surface area on the bottom and there seems to be lots of drag due to that.