The snugglesafe's are better than a hot water bottle because they stay warm all night. They are quite hard rather than squishy as well. This is the rack (small towel rail!) I used to attach to the outside of the bars - the snugglesafe sits in it perfectly so it can be up against the bars on the outside in the nest/house area for example.
https://www.amazon.co.uk/Kitchen-Cab...oor+towel+rail
This is the Snugglesafe itself. Coco's idea of putting it under a tank/cage raised on books might work better for tank as no bars
Either way it's an additional warmth source and the room still needs to be at a reasonable temperature as well, so a small oil filled radiator left on overnight near the cage/tank is a good idea too (not too close, but close enough that it warms that area as the small ones don't heat the whole room that well).
I'm using both at the moment - oil filled radiator on overnight (and during the day as well partly when no other heating), the snugglesafe at bedtime, and putting extra paper bedding out every night (and it is all being taken each night).
https://www.amazon.co.uk/SnuggleSafe...ds=snugglesafe