Try posting any plans/ build you can find anywhere on the net. So some members can comment. But let me give you some important pointers. Strip all the non metallic parts of the fridge. They are not good conductors. Paint inside out with thin coat of flat black. You may want to scrape the original paint, paint is also poor conductor of heat. Hot air rises and cool air goes down. Don't think of collecting the suns heat on the air alone collecting it on metal is far better than just trying to heat up air or any material with low thermal conductivity like
wood or plywood. If air is cool in your place. You have to cover the whole thing with glass or transparent plastic so that the breeze wont rob your fridge of the heat collected. put some space between the metal wall and the glass. at least half inch to 1 inch space is ideal. just post anything you can find we will try to help you.