This actually worked great. Scrap plywood + shoelace. No rocker, no finesse. Zero dollars. Had a blast out there today, to my surprise.
The only thing I'm wondering about is whether the fancier versions of these things made by Hess et al are easier to use when one is paddling. This thing is half inch plywood, so it's a little clunky in that respect. But I was amazed by how much it helped just holding a line in the tube.