You could fasten a vertical piece of wood to either end od the window ledge (screwed into the Wall obviously),and then fasten chicken wire across the space, this would be inexpensive, should work, but will probably not look too great. I would suggest that you buy some rectangular plastic planters, and screw them to the window ledge, fill with compost, and some foliage, evergreen plants, such as miniature conifers. In spring and summer you could add bedding plants for colour, and fragrance. If you then found people interfering with your display, you could add the chicken wire as above, you will probably be able to carry out maintenance from inside your flat. Hope this helps.