Put a vacuum pump on that tonic water and evaporate the water content from the batch until it is sufficiently stout to your liking. Use a strong stainless flask for the low pressure working vessel to avoid implosion. Might need supplemental heat to keep the tonic from freezing in the flask!
I actually have one more trip’s worth of caps. After that I will rely on God. Everything that has happened to me and to the people I am involved with says that I am an agent, not a principle and that I am protected, at least until the task is finished.