Tragopandemonium made this absolutely magnificent lava lamp costume for Halloween. They posted a gallery of the creation process at Imgur:
I thought this would be an easy costume to make (lol), but right off the bat finding the right plastic was a whole quest. First I tried thick marine vinyl, but it was too floppy and over a few days the cone sagged and collapsed! Finally I found a local plastics company that sold big sheets of polycarbonate. (Perfect!) […]
I began the lava garment by chopping one boob off of a bustier. Then I built up the remaining boob with craft foam and bra pads for a more lava-globb shape.