@FusionChefGeoff this is my plan for experiment #4 today of the low carb frappuccino. Note that it isn't the finalised recipe yet but I can't afford to have more than one a day as each is approximately one million calories. 
Ingredients
Two level teaspoons of good quality instant coffee (decaf in my case) made up into a liquid with a little boiling water, preferably allowed to cool in the fridge ahead of time but it's not essential. Or you could use a shot of espresso if you have an espresso machine.
100 ml whole milk (you need whole milk because you're blending it with ice so semi-skimmed gives the wrong mouthfeel)
50 ml double cream (this adds sweetness without adding sugar)
250 ml crushed ice
2 tbsp unflavoured whey protein powder (probably optional but without it it's practically all fat)
1/2 tsp xanthan gum (without this it won't blend smoothly and will be more like the texture of a Slush Puppie so it won't go up the straw)
1 tsp vanilla extract
Pinch of salt
50 ml whipped cream (I use a cream whipper for this but you can get "proper" whipped cream in a squirty can in the supermarket)
Method
Put the ice in a jug blender with the coffee and pulse to break up the ice into small pieces.
Add the rest of the ingredients.
Blend for 10-20 seconds on lowest level then 10 seconds on high until smooth.
Top with whipped cream
Nutrition
490 calories, 7g/7% carbs, 17% protein, 76% fat.
If you skip the whey powder it's 410 calories, 7g/7% carbs, 5% protein, 88% fat.
For a lower calorie option I'd skip the cream on top and reduce the whey protein to 1 tbsp which would be 340 calories, 7g/9% carbs, 15% protein, 76% fat.
So it would depend on your goals. If you're overweight and actively trying to lose weight then the last variation would be best. If you're close to your target weight and actively trying to lose weight then the second option would be best. If you don't care about the calories and just want the nicest drink then the first option would be best.
Anyway, more experimentation needed, and it sure has been enjoyable experimenting!