Just for the hell of it I went ahead and cobbled together this trunk rack attachment out of pvc tubing, some U-bolts, and pipe insulation. Total cost was about $15.
I strapped the tubing into the trays where a bike's top tube would normally rest:
I set up the bar spread so the skis nestled in between the mounting points. Note that the weight of the skis/poles falls mostly upon the metal bars of the trunk rack and not on the pvc. The pvc adds stability further out toward the ends:
I secured the skis and poles with the rubber straps then with three separate nylon tie-downs. It all felt secure so I went for a test drive around the neighborhood. I rechecked and found no slop. I said, "What the heck!" and decided to use it on our journey. I tucked the kids' skis in the trunk though, and pierced the adult skis through plastic trash bags to protect the bindings from the elements. The trash bags got thrown out while at the resort so the bindings went uncovered on the way home, but the weather was clear and there doesn't appear to be any road grime on them at trip's end:
Three separate tie-down straps augmented the rubber straps:
After about 900 miles and 19 hours, at highway speeds (some of which was sustained at 80+ through Idaho) and through variable weather, the entire setup held fast. Even at a brief stint above 100mph there was no instability and barely any tip flap (ha ha). No pieces fell off and it doesn't appear that anything even shifted. Not the prettiest contraption but it got us there and back for a terrific vacation with 31" of unexpected snow!