Paint Chipping

A piece of plastic hosepipe, sliced down the side might do the job - glue it on with a small amount of silicon sealant (don't worry you can get it off if you need to).

The other way would be to wrap something around the rail in the style of bandages - use some clear PVC (Vinyl) or similar cut into strips. you could use adhesive pvc tape, but the glue tends to work it's way out and gets sticky and messy.

If you are struggling to see what I mean then have a look at a few pictures of sailing yachts in harbour (google image search works well) - you will see every different possible way of covering up a railing.

Hope this helps, I found no sign of a purpose designed piece for this crib online :)