The app I found that works best is Icon Slate, by Kodlian. It costs $5 in the Mac App Store and is worth every penny. It’s very simple. First, create your 16 × 16 and 32 × 32 px icons in the image editing application of your choice — Acorn, Pixelmator, or Photoshop probably. Export each icon size to its own PNG file.
Icon Slate worked well for me, too.