What about using some parallel PVC pipe, say 1.5 inch diameter? Drill some holes in them and thread a rope through a smaller diameter pipe, then through the two larger ones (think of a ladder). The rope prevents them from sliding away from each other and the smaller pipe keeps them from collapsing. The nice thing about PVC is that the kayak will slide very easily.
Ha ! That is exactly what I did for the 2-3 times a year I'm up at my In-Laws. They're on a very rocky waterfront.
Think of the whole contraption as a PVC Ladder ... crossbars are the steps. on the steps, I put pool noodles over them so they either roll on the PVC or the yak slides easier. I also used some of my older climbing gear rock stoppers to secure the ladder to the rocks. When I'm all done, the whole thing folds away nicely into their shed for the next time I'm up.
This year, I plan on hauling up some nice 4" PVC pipes (as long as possible) and making a dedicated launch with them