Windowsill fly trap

Edit:
The tabs on the lid are too large. Working on it now.

Every winter it will warm up above zero for a couple of days, then out come the flies! And they mostly wander around the sink window, hence this remix.
The window is an old (1974 model house) double-pane slider, and this is designed to slide onto the channel to hold it in place. It may work on other types, but this is what it's made for.

fritte2000 recommends some water, vinegar, sugar and dish soap. I'm going to experiment with proportions.