I tried a can collector but it took time.  I came up with this which also works well and is a quick build.

This is foil faced foam board, sometimes called Tuff-R.  Mine was free from building sites.  This is the side facing the sun, painted black using barbecue spray paint.

Here's the back, facing into the room.  The slots are covered with black plastic trash bags.  They were originally intended to act as reverse flow constrictors.  They work OK but could work better since they don't like to stay 'flat' but as they are now they add some privacy.  I figure this set up might add a smidgen of R value since the only covering to this window is aluminum blinds.  I've seen 130°F in the collector during the winter once the sun gets lower in the sky and the heat pours out!  As windstuff pointed out, you lose most of the light.


You could probably do this with thicker cardboard also... I might have to try that.

If you like cans and want to try liquid...



I've been playing with this since last spring.
Two aluminum cans silicon'ed together and spray painted black.  The reflector is an old snack chip bag turned inside out glued to a 5 gal. bucket half from old VAWT experiments.  The bag has a clear coat applied that started turning opaque in the sun.  After a few weeks this coat finally started flaking off.  Once it was gone the thing really started reflecting and the heat came up a lot.
During the summer, a few hours will bring the water up to 165°F.  Yesterday with the temperature at 49°F and a bit breezy, two hours of clear sky gave me 24 oz. of 127°F water.  Tea anyone?  I've also tried using 2 liter plastic pop bottles, dark green, which also heat up quite well.  I need to get some plastic spray paint for the pop bottles and give that a try.