My son wanted to be a light blue Pac Man ghost for Halloween. I took a large piece of cardboard and cut it to shape (I looked at images online to figure out the shape). Then I painted it blue and added the eyes. I made straps out of heavy clear duct tape. Take two pieces of long tape and stick them together (with both the sticky sides together) to create a heavy duty strap. I criss crossed two straps and attached them to the front and back sides of the costume.
To give it a little something extra I made a headband with black and yellow posterboard and then used white Avery circle label stickers for power pellets. My son won 2nd place at his schools Halloween costume contest.