On a whim a couple of years ago I bought a Reduce COLD-1 stainless double insulated thermos from the discount bin at Kroger for a few bucks. One of my best bargains. It's been refreshing on many long hot summer rides the past couple of years. Occasionally I'll use it to hold hot coffee or tea in winter, but mostly use it in summer.

It's the right diameter for a bike water bottle cage, but lacks the dimples that mate with the retainer fingers on some cages. But that hasn't been a problem. The textured finish grips the aluminum cages just fine. If anything I need to relieve the tension on my aluminum cages to retrieve the bottle. So far it hasn't bounced out of my much looser fitting plastic cages on my road bikes (I use Trek plastic cages on one road bike, 50Strong on another, both nearly as light as carbon fiber at a fraction of the price).

If I'm anticipating rough roads I'll loop a wire twist tie from a bread bag to retain the tethered cap to the water bottle cage. Probably unnecessary, but I prefer to be sure it's secure in group rides. I don't want anyone to fall because my water bottle popped loose. That already happens too often in group rides, although fortunately I've seen few falls.

I usually half fill it with electrolyte solution and freeze it. The next day I'll fill the rest with plain water. If I use the original thermal cap the frozen mixture will stay frozen for up to 12 hours even in summer, gradually dissolving as I add fresh water. If I use a drinking cap with a Polar or Camelbak style nipple, it will thaw much sooner but will still be icy cold for hours.

Typically I don't drink from it while riding. Instead I reserve it for rest breaks. As my Polar plastic insulated bottle warms up I'll transfer water/electrolytes from it to the COLD-1 thermos. So there's always something cold to drink for as long as the ice remains.

Unfortunately that particular Reduce COLD-1 thermos has been discontinued, but I see similar size stainless thermoses by different manufacturers in local stores and online.