I tried the Splenda/Kool-Aid/Knox thing several times and could never get the proportions to suit me. It was either too weak, too strong, not sweet enough, or too "gummy." I recently found this recipe using DaVinci sugar-free syrups, and I haven't tried it yet but I'm anxious to do so.

Combine in a shallow dish:
2/3 cup water
2 packages Knox unflavored gelatine
Stir to mix. Allow to sit for 5 minutes.

Combine in a large measuring cup or small bowl:
1 cup DaVinci Gourmet Sugarfree syrup (pick your flavor!)
1 cup water
1/2 teaspoon lemon juice

Microwave until almost boiling. Pour hot syrup mix into softening gelatine and stir well. Place syrup/gelatine mixture in microwave and heat an additional 2 to 3 minutes. Remove from microwave and stir well, being careful to stir all the way to the bottom of the bowl. Repeat heating and stirring for 5 to 8 minutes (until ingredients are completely mixed.) Add lemon juice and stir again. Allow to cool until it reaches room temperature. Refrigerate for 6 to 8 hours.