I like to use the Crystal light drink packets and the Gaterade 0 packets. They are about 5 calories so I prefer them to just drinking water. I created a holder that goes on my water bottle to hold the packets. You can just scale this up or down to fit your water bottle. I made the ring about 6mm thin so it wouldn't interfere with closing the bottle and to make sure the bottle didn't leak. I also designed this to print as a single part. Only the lip needs supports, as the top on most printers can be done via bridging.