There are a few things you can do to help it.
1. First, be sure your smoker is out of the wind. I assume it already is if you have it in a shed.
2. Do not use an extension cord. If you must use one get a heavy duty cord that is made for a large appliance, at least 12 ga.
3. Pour hot water into your water bowl puck catcher.
4. Preheat a couple of bricks wrapped in foil in your house oven and put them on an empty rack.
5. Further insulate the outside of your smoker with something like a hot water heater blanket.
6. Keep the top vent open. Do not be tempted to close the vent completely to try and hold in the heat.
7. Add a second 500W element or replace the original with a finned 900W. If you do that you'll also need to add a PID controller. There are "how-to" guides available on how to do this.
Here are some more ideas:
When/if it gets cold out...Insulated Blanket/CoverNormal temps...