Look up the house recipe from the Whimsical Bakehouse and make it with all shortening. If you can get high-ratio shortening (sometimes you can find it at specialty grocery stores like Trader Joes or Whole Foods) it will be less "shortening-like" than if you use Crisco. I myself do not like any frosting made with vegetable shortening, but this recipe I can at least handle a sample of it. Since you are having it outside, use the lesser amount of water. Beat at high speed for the full 20 minutes for maximum volume. If you need the batch size for a 20 quart mixer, PM me or post here and I'll get it from work.