Dynafit Universal Bottle Holder

Sometimes you can feel the wind catch this bottle holder when descending quickly, but other than some aero improvements, this bottle holder doesn't need much. Has kept bottles warm in -5 or so, and the elastic keeps bottles in rather than on the slopes. If you want a brighter color, consider buying an RC20 pack in the color of you choice and swapping (I got an orange from a PDG).

I have an older version, but as you can see in the attached picture (while taking a break from some intense touring in my Millet Touring Intense pant), essentially the same design, even with the same piping.
To attach it to other packs, might take some ingenuity and zip/cable ties, but well worth it, given the convenience in the field of both drinking and refilling your bottle.
On this particular tour, all the usual running water sources on Mount Adams were buried in the deep snowpack during the entirely of our 9,500'-vertical tour, despite the mid-July date.
Fortunately I could instantly just grab a snowball's worth of slushy snow and toss it into my water bottle, therefore extending my original water supply.
The elastic retention cord keeps the water bottle stable: doesn't bother me at all skiing with a bottle there, and I've never lost a bottle either.
(And in case you're wondering what I'm doing in the picture while wearing my embarrassing Petzl Sirocco helmet, I'm explaining to another Petzl Sirocco wearer that a new less-embarrassing is coming out for the following season -- yes, really, I was doing that!)