If you have a metal paint roller tray--the kind that is made to slip over the platform of a step ladder--just drill a few holes in the bottom to let water drain, slip it over the deck rail and fill with seed. Works pretty well, although you have to careful to keep it very clean.
We also have had in the past (but not currently) a mesh-bottom platform feeder with brackets on one edge to slip over the deck rail. It was store-bought, but I haven't seen one for years. If anyone ever sees one for sale, please share the info.
Even a mesh-bottomed platform feeder will not prevent snow buildup, though, so you still have to do a lot of cleaning with either feeder and be very diligent when things thaw to keep the seed dry.