I made a shelf for flowers by the window but the windowsill was slightly slanted inwards, so I made a small wedge to make up for the difference. The shelves now lean towards the window (the walls of the shelves go on each side of the actual window glass)

The shelves were made of 18mm thick planks. The wedge fits them.

Upload this to e.g. tinkercad and stretch to fit yours!!

UPDATE 23.3.2015

Wedgestravaganza!

I made 2mm and 1mm versions of the wedge and renamed the original to wedge3mmthick.

Now I have more variety for my window shelves!

UPDATE 28.5.2015

Photos depicting the 1mm and 3mm wedges.