Just off the top of my head if there weren't thick plant holders out there perhaps scouting the bike hanging racks at the Bunnings equivilant in your area is would help out - I have something that could support 2 heavy containers and easily fit over your wall which I hang my bike off in the garage.

Sorry about this! I actually submitted an extended version of this question but I guess this version made it through instead.

The details are as so: My railing is 7 1/8 inch deep, and is 4 7/8 inch on the sides. Most balcony plant holders fit a 2x4 or 2/6 railing. My railing is very thick so its been hard to find anything. I've been a bit scared to put pots on here since its quite windy here!

Thanks Captain's Catfish for the advice. I'll have to look into other people's experiments with bike hangers. I never would have thought of that!