I designed a shelf that hangs from my window sill. The outer frames are more than strong enough to hold this 4lb (1.8 kg) fan, but I added a brace in the middle mostly because I ended up using cardboard instead of printing out the shelf itself.
The clearances are designed specifically for my window, but it's fairly trivial to extend the horizontal and vertical members to get more clearance. If you decide to print this, do keep in mind that you'll want thick walls, some infill, and you'll want to arrange the parts such that the force of the objects on the shelf won't be pulling to delaminate the prints.
WindowSillFrame.stl | 132.3KB | |
WindowSillShelf.stl | 136.4KB | |
WindowSillShelfBrace.stl | 83.5KB |