Man, be careful! You do not want a live electrocution as part of your display.
For electric connections in the yard we've used gallon ziplock bags that we've cut holes in the sides of to feed in each end of the plugs. Once the ends are together, seal the holes up with glue (we've used Elmers) and zip close. After Halloween just cut up the bags. Everything's stayed dry so far - good luck with your set up!