I've used 21mm ply, cut maybe 40mm wide and as long as you need, for how ever many rods you have. Drill 8mm dowel facing slightly erect (most descriptive word, sorry) so the rods don't roll off. Glue dowels in and make one mounting hole 1/3rd from the top. Presto, rods rack.
If you have long surf robs, rather buy a length of dowel and cut to fit thinker robs. If you have a full wall spare of course. My surf rod is hanging from the rathers from my pencil bait wires/draad. Could also make a proper bracket to hand from.