I believe you would need something to take up the distance created by the 30° angle. You'll need a surface the planter can rest against that is at right angles to the floor. You could take a scrap of something like 50mm thick timber and screw through the strap of the planter so it is fixed to it. Or perhaps cable tie the piece of timber to the fence. The straps curving backwards away from the pot probably aren't helping the situation either. You could try bending them back towards the pot as they appear to be made from quite thin metal.
Is this for aesthetics or because you are worried that the plant is not positioned perfectly upright? The plant most-likely doesn't mind the angle it is planted at, I have quite a few plants at such an angle doing fine.