I use these linked to below on my round iron railing. We had to hang then unconventionally, though, and that leaves a few inches of metal sticking out in the opposite direction. Last year I hung them facing inwards on my balcony so I didn't see that part. This year I will be hanging them on the outside of the railing (to get more room) and I am worried about that metal sticking out towards us.

Here is a link that might be useful: railing brackets