Checkout this how-to video to make a simple focusing mask that works great. When not helping you to focus your camera, it also extends your dew shield to keep dew off of your lens.
The materials you need to make this focusing mask are:
1 Foam Sleeping Pad
1 Roll Of Masking Tape
2 Wood Kabob Sticks
All materials can be easily found at your local Wal-Mart for cheap.