Summary

I was looking for a place to put my Basil plant and I did not want to put up a large shelf and block the window.
This shelf uses two T-nuts to grip onto the slot on the edge of my vinyl window (horizontal slider). If you have a similar window you might have to remove a trim / cover piece from the slot to gain access. The customized T-nuts come from https://www.thingiverse.com/thing:1729060 . Your dimensions for the nuts will likely vary and so you should measure the opening. Mine was 22mm and that is the size of the T-nut here. You can customize the L/R alignment of the screw holes on the shelf by using Open SCAD and the shelf SCAD file here. I found some very short wood screws to assemble it which are simpler than machine screws. Assembly is tricky as I had to support the back of the T-nut with my pinky finger while I drove in the screw. A large window can support multiple shelves which can be slid up or down by loosening the screws. The sliding window will still open to about 90% of its width