Yes! I finally found a solution for a perfect balcony cat bed!
All you need are adjustable holders for jardinieres (is this really a word??) and a basket / box which stands rain. You can see black balcony trough holders under the board next to Kajsa. In Finland, all bigger stores for garden supplies and plants sell those; they cost about 4€ ($5). So instead of using them for flowers, the holders keep a basket. This construction also works for other kinds of banisters, for example handrails in staircases.
My cats didn’t like sitting in the flower pots you can see on this picture, so I gave up and used them for flowers instead. I guess they were too narrow. Tessa likes the board as well, but Kajsa needs something to hide in.
On a side note: the board consists of left-overs piles of weatherproof wood I bought for our catio. I wanted to have a sideboard for my coffee, but great that Tessa likes it as well.