I recently bought Feller 2-channel shutter switches from Hornbach @ ca.116chf each. Everywhere else had them for around 160chf each.
Just a shame their online shop needs to up their game.
Hornbach is suprisingly good value, often much better than Galaxus.
Seriously, since you can let the plates rest on the inside of the balcony, the zip ties will have just to keep the plate vertical and prevent it to move.
Go with it and don't worry about it.
On the other hand...transparent plexiglass? Have fun keep it clean, especially with the bars in the way