Diet gelatin with Splenda is just too hard to find,
so I just tried making my own from drink mix,
for the first time,
and it worked well,
so I am sharing how I did it.

In a 1 quart pyrex measuring cup, place 1 cup water.

Sprinkle 4 envelopes unflavored gelatin over the top.
I used Knox unflavored gelatin
and the box seemed to suggest that one envelope
deals with one cup of liquid.

Sprinkle in drink powder
appropriate for 1/2 gallon of drink,
and sweetener if the drink powder does not have it.
I used 4C Totally Light Cranberry Pomegranite
which already has sweetener in it.

Mix liquid until all powder dissolves.

Heat at least 3 cups of water to gentle boil.
Add boiling water to pyrex measuring cup
until volume reaches 1 quart.
Stir until combined.

Place in refrigerator until firm, possibly overnight.

The end result was good,
but maybe stiffer than needed.
We might try less gelatin next time.
We have not tried a jello mould, but it should work.
You don't have to use pyrex, but
make sure your bowl can handle boiling water.

I don't think I've ever seen 4C drink mixes around me, but that flavor sounds really nice...may have to go on a treasure hunt!

from the Knox box:

Directions: One envelope of Knox Gelatine will set 2 cups (500 ml) of liquid or 1 1/2 cups (375 ml) solids.

Your 1 quart (or very close to 1 Liter) would have set to 'normal' gelatin thickness with 2 envelopes.

Crystal Light or any houseband/generic sugarfree/light drink powder will work in this recipe, too. I've got a lot of great flavors of drink mix in my cupboard I'd like to turn into gelatin dessert...

There's a sugarfree gummy worm type recipe floating around here and on the web that uses double or slightly more than double the amount of knox with unsweetened Koolaid -- that works well, too!