I wanted to make my own also but just wasn't sure either. If things go like last year and I get the stinging catapillars on my Sweetgums I will probably make up my own just because the trees are so big. I found a few homeade formulas from reputable people that I'm told will work.
The key seems to be a bit of oil (sunflower-canola-cooking). I have also heard to only use Dawn or Ivory soap but that's probably personal preference. Some also say not to use anti bacterial and pure liquid castile soap is what the commercial venders use.
How to Make Your Own Organic Insecticidal Soap and Keep Your Garden Chemical Free
anyone make their own insecticidal soap spray? - Square Foot Gardening Forum - GardenWeb
I will post again with the results of my homeade recipe in a couple weeks when I can "beat up" on the stinging catapillars.