This has been a good thread with a lot of ideas for adding a 2d water bottle cage to an old bike. Another possibility to keep in mind is the zefal magnum one liter bottle. It holds 33 ounces which is significantly more than a "standard" water bottle which tend to come in at around 24 ounces capacity. There are some online complaints about leaking but the new ones, in my experience, do not. The only downside is that they are tall and they can be difficult to refill in some sinks.