This year my son asked to be an alien in a UFO. Of course it was after we already had a pumpkin costume ready for him. This is what I came up with after a few hours in the shop.
Step 1: Materials
For this costume you will need;
Gorilla and or Duct tape
Aluminum Foil tape
Roll of Aluminum Foil
Led light string
Lars Scanner kit (optional)
Strips of material for straps
Hot glue gun
Step 2: The Build Part One
Begin this costume by laying out a large sheet of cardboard. I used a refrigerator box left over from a rocket ship I made last Christmas. Draw two circles the diameter of your choice. I tied a pencil to a string and measured out 16 inches. Held the string in the middle and made my two circles.
Next, carefully cut out the circles and using your same method cut out a center circle to fit your little aliens waist. Mine had a radius of 5 inches.
Cut a strip of cardboard approximately 3 inches tall and long enough to go all the way around your inner circle. This will determine the height of your ship. Using duct tape attach the spacer strip to the bottom disc.
Now cut the top disc into two halves. This will allow you to slightly curve them inward making a cone shape. Sorry I don't have a picture of this step but I'm sure you can figure it out or just score the cardboard enough to make it work. Duct tape the ends together in just a few spots. You will need to take them apart again to install the lights.
Step 3: The Build Part Two
After you are happy with the shape and trimmed the edges a bit. Evenly measure out spacing for your lights. I got a string of 18 multi colored LED's from Target that run on 4 AA's. Approximately 3 inches from the bottom came out pretty nice with a few extras at the end. Insert and secure them with some more duct tape from the inside. The LARS scanner I put in the front is from ThinkGeek.com and I just used a little plastic case and hot glued it in.
Next wrap your UFO in tin foil the best you can. I would tape one end of a piece down and cover over it with another. Clear scotch tape takes care of the seams and the aluminum foil tape will trim everything else out.
Tape your control boxe(s) into the middle area where ever they fit and make sure the switch is easily accessible.
Next, measure out two pieces of material to fit your alien so that the UFO rests around their waste and cover in Aluminum foil tape. Attach any way you like. I used large staples and plenty of hot glue. Cover up any ugly spots with foil tape as needed.
I didn't like the lights just sticking up in the open, so I "procured" enough ketchup cups from the local fast food place to cover them up. I just used hot glue to secure them and they really defuse the light well.
Step 4: Finished
Hopefully this instructable was good enough that your UFO looks sort of like mine. I covered a toy helmet with foil and made some antenna to complete the costume. All your little alien needs is some green face paint and a bag for trick or treats.