poncho

Measure the height of your figure from either back of knees or waist to base of neck, depending on length wanted. Pick the material that you want for the poncho - you probably want something thinnish - and use said measurement as the radius of a circle. Fold into quarters and cut the center off - you're looking at something between slightly wider than the width of the neck to halfway along the shoulders, depending on fit. Unfold, and voila - one poncho.

Another design is to use a diamond instead of a circle, making sure that one of the points is in front.

I use the same circle method for capes, only with a slit from an edge to the center, or cutting out a section for a gap. For example, Batman's full cape would just have a single cut, whereas somebody like Superman would be a halfcircle.

Maybe a small piece of a tarp or painter's dropcloth?

a piece of cloth with a hole cut in it. lol
then you can get whatever colors and have a different material on the fig to make it pop.