The cotton aerogel - insulated military canteen offers better heat insulation performance compared to commercial insulated water bottles such as FLOE bottles, and is highly comparable to that of vacuum flasks. (sciencedaily.com)
Oven does just as well, or warm it up and put it in a wide - mouth vacuum flask. (davidlebovitz.com)
When I placed a wide necked vacuum flask filled with water in the sun yesterday (with the lid off) the temperature of the water rose from 19.5 deg.C at 5:08 am to 29.1 deg.C at 1:53 pm while the air around it rose from 19.0 to 31.9 deg.C. (carlineconomics.com)