A liquid mask painted on is probably your best bet in this situation. Because you have to paint the inside of the sills next to the window you will have to be very careful to not let any of the mask get up against them. The good liquid latex masks that I have used seem to start to set fairly quickly so it makes it difficult to do fine exacting work. Once it starts to set you can't touch it with the brush or it makes a mess and trying to trim it after it sets also results in a mess. The good thing is that if you do mess things up, just let it set a few minutes, pull it off and start over. Also some masks tend to want to bead up when applied to plastic in a thin layer. It will probably take a bit of practice applying it to get the feel of it and you may want to thin the mask if it is fairly thick - but too thin and it will want to run beyond where you want it to go.