We had a basil plant on the window ledge in the kitchen, that didn't really fit, but fit well enough that we thought we could “balance” it there. Of course, we knocked it over constantly.
This is a basic platform that I made to make the plant a little more stable. It extends a couple of fingers down, and uses some pads against the wall to help reduce the risk of it tipping over. The plant is much more stable now, and lives happily in the sun on the ledge.
I would not recommend this for anything valuable, or breakable. Plants in the plastic pots are perfect.
**Please see pictures for key dimensions.**
There are two lengths of fingers, 3" and 5". Pick the one that at least clears your window casing. If either will work I would go 5" since that seems to be a bit more stable than the 3".
From my testing, it seems to work pretty well as long as you have ~half the platform on the ledge still.
Assembly:
Prints with everything you need, no tools to assemble.