I've got a Pat's Backcountry Beverages setup that I don't seem to ever have the opportunity to use. The thing is pretty cool, and if you swap out one of your nalgenes for this bottle the weight penalty is minimized. Full disclosure, you need to use cold water. Doesn't carbonate well with water you have been carrying in the mesh pocket of your pack in the sun all day.
Starter kit with an odd assortment of soda packets:
http://www.patsbcb.com/product/carbonator-kit/
Two boxes of the carbonation packets:
http://www.patsbcb.com/product/eco2activator-12-pack/
Two of the Black Hops four-packs:
http://www.liquorama.net/pats-backco...te-4-pack.html
Two of the Pail Rail four-packs:
http://www.liquorama.net/pats-backco...te-4-pack.html
This was a little over $100 new (especially with the high shipping on the last two items which you have to buy separately).
$70 plus actual shipping.