For my frailer piggies, I prefer to cool my microwaveable pads in the fridge and put them underneath their favourite snuggle place. You need to replace the pads with fresh ones every 2-3 hours (I always have a second lot going for that reason, so I can switch at need).
It is not for people who are out at work all day but it is a gentler way of cooling down oldies than a frozen bottle.
Key is however not so much just sticking a frozen bottle in the cage but either keeping their room as cool as possible or moving them into coolest room of your accommodation. See a frozen bottle as a last stop measure because when your piggies snuggle up near it, they are already overheated - which is something you'd rather avoid in the first place.
The other important point is that while piggies will gradually adapt to higher temperatures; what is a killer are temperature jumps of over 10 C, especially for the frailer (old, ill, newborn or pregnant/nursing because of their weakened, weak or overloaded immune system). This also includes the quick change between day and night temperatures for guinea pigs that are not used to them. Guinea pigs used to stable indoors temperatures should not suddenly be exposed to temperatures over 25-30 C. It is also important that temperatures in full sun are
much higher than those in the shade (which are used as guides). And that a brisk sun-heated breeze can still kill piggies in the shade or that are exposed to it by a fan being placed next to an open window in full sun.
We sadly see those kind of cases every summer.
I started the day in my dual aspect lounge which has sun on it all day long with 21 C and still have only 23 C (at 5 pm) thanks to airing thoroughly first thing in the morning and then keeping the french doors on one side and the bay window on the other protected with old bed sheets, which I have been dampening every few hours as we are coming close to 30 C here.
A can, which can be pumped by hand to provide the necessary pressure for spraying, is what I use for the front, including to water down my hub's unprotected office window; this is something that can be still used during a hosepipe ban. I haven't used any extra electricity and very little extra water to dampen the sheets in a sink. There is a fan if really needed but I prefer not to.
We now also have heat reflecting film on most of our upstairs windows; it takes about 1-2 degrees off on a really hot day.