I just saw an "alpine mug" at camping survivial.com that fits over a 1 qt. Nalgene bottle. Its a 16oz stainless steel cup with folding handles. I'm thinking about getting it for myself. I think 16oz is a good size.

I'm glad I kept looking. I just found an "oilcamp space saver cup" for 5 bucks thru bizrate.

I use two water bottles of different types for dayhikes. First is a regular nalgene bottle. Second is a British canteen with metal cup. The brit canteen is a liter, holding a little more than an american one. The mug is bigger as well. The combo has worked well for me. The black canteen also has the advantage of warming up in the sun, allowing you to melt snow (provided it is sunny) by packing it in & letting it sit.

Does anyone know which, if any, of the Snow Peak titanium mugs fit over the end of a nalgene bottle? Thanks!