Steve, what are the tolerable temperature limits for storing your sister's insulin and glucose? It occurs to me that a small soft-sided icechest would likely stay in the zone (not too cold; not to warm) if you put some dry ice in it. You would not need much. Dry ice (frozen carbon dioxide, I think) is available in most grocery stores in the US, so I suppose it's plentiful there to. If you could depend on buying more ice each day on the route, that seems like a doable deal. You'd want to test this well in advance of the ride, of course, but I'd guess that about 750g of dry ice would keep a small ice chest within limits for a good 8 hours -- provided you don't open it more than absolutely necessary. The nice thing about dry ice is that it makes no water as it melts. It just goes straight to gas. My soft sided chest is closed by a zipper so the it would likely breath enough to handle the volume of escaping CO2 gas.
**The freezing point of CO2 is -109.3°F or -78.5°C.
**As a general rule, dry ice will sublimate (go from frozen to gas) at a rate of five to ten pounds every 24 hours in a typical ice chest.
**Dry ice requires special precautions when handling. It is extremely cold, requiring proper insulating gloves to handle. It constantly produces carbon dioxide gas, so it cannot be stored in a sealed container as the pressure buildup will quickly cause the container to explode.