[Fat Charlie]

If you aren't going to insulate all the walls, start with the north, then the east. In some parts of the year, the south and west can provide valuable heat.

Rig an awning on the south side: cloth from the eave to the fence. You don't need to insulate against the heat, you simply need to keep sunlight off the southern wall. In the winter, allowing sun to hit the wall can be helpful, especially if you knock together an air heater like at builditsolar.com.