In a moment of weakness and dire thirst, I recently decided it was worth the buck and a half to get one of these delicious MetroMint products. They really are very good, but when I got home and still had a partial bottle left over, I tried a little (very little) experiment and made my own version. In a side-by-side taste test, I couldn't tell the difference. Since then, I've been making this "recipe" pretty regularly, even to the point of washing out the bottle and reusing it for the rotation (one batch there and another in my beloved water bottle - one always cooling in the fridge and the other travelling with me).
Note: the only remotely "hard" thing about this process is measuring one drop. I do have a bottle of doTERRA's Wild Orange with a convenient dropper lid, and I've tried a variety of things with the mint including covering the bottle opening with my finger, tipping it and then just sticking my finger in the water. "Home Economist Sue" does not recommend this. "Practical, I'm only making it for my own consumption Sue", does.