The secret to making a scientifically perfect gin and tonic is apparently dry ice:
Using the same ratio of 2.5 parts gin to 4 parts tonic, combine 5oz of gin, 8oz of tonic, and 2t lime juice and pour into a tivolo tray. This should make 13 1oz cubes, leaving two holes for you to fill however you like. Put the tray inside an insulated container that has at least 5lb of dry ice in it and freeze for 1 hour. These cubes will not be easy to remove from the tray, and I recommend using a towel or cutting glove.
Two questions remain: What is a tivolo tray (tivoli tray?), and where do you buy dry ice? The answer to the first is I have no idea and just spent more than 30 minutes trying to figure it out, and the answer to the second, as far as I can tell, is through this freaky dry ice directory.