I was using a paper towel as a coaster for several months before I realized that I possess the technology to fix my problems. (Well, not all of them...)

I designed this super simple coaster to be more or less molded to the underside of a standard Nalgene bottle. It fits fairly nicely, keeps the bottle centered, and helps resist knocking it over at least a little (I am not responsible for spills). I also made a lattice cut into the part that lies under the base as a means to drain away condensation if you have ice water that'll sweat.