Be Safe: Planning for electrical safety in your Sukkah.
This is an example of a weatherproof box that can be used wherever two electrical cords come together. Example, plugging a light fixture into a power cord or extension outlet. It is called a Sockit Dri-Box and it is available in three sizes from amazon.com.
If you have just a electric cord and an extension cord to put together, you can use a Stanley Extension Cord Safety Seal online from Amazon, Home Depot, or many local hardware stores. It runs around $10.