A company called Flogos (an amalgamation of floating and logos btw) has created a machine from a repurposed snowmaker that creates floating clouds in any shape. The “clouds” are actually made of a soap based foam and lighter than air gases such as helium.
The Flogo generator releases one flogo every 15 seconds and the clouds last for a few minutes to an hour depending upon how the mix is formulated for your needs. The soap used is environmentally friendly and will usually evaporate in the air. Flogos can travel up 30 miles and 20,000 feet up.
Flogos uses a computerized stencil system to create outlines in whatever shape you desire. Currently available in 2-4 foot sizes, but a six footer is coming soon. The machine costs about $2500 a day to rent.