Some people also give their cats a tiny bit of cheese or plain yogurt.
These are not 'carb free' as they contain lactose. But we are only talking about a tiny amount here, used as a reward treat. And since you're testing, you'll soon pick up how they affect your kitty's BG level (if at all...)
I'm sure there's someone here who gave their kitty shrimp as a test reward.
In the early days of testing, when we're trying to get the kitty used to the process (or even to like it

) I think it's OK to not stress tooooo much about whether a treat contains a little bit of carb, or indeed, whether it's a fish treat (knowing that fish should be limited in the diet). Let's just get them on board with testing, and we can refine the choice of treats later.... (Just my 'ten cents'....)
And some cats end up being just fine with no food reward at all. Some are happy with a cuddle or a little grooming session.
....And there may even come a day when you can test your kitty in his sleep...

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