Hi, I use snugglesafe (zooplus.co.uk has them pretty cheap) with my piggies outside. They're great and if you place them in box stuffed with lots of hay they'll last even longer compared to if just left exposed.
Just create boxes (plastic,cardboard,soft) with hay in as this way they'll be able to keep warm (smaller area to hold the heat).
If your cage is kept on the floor place it higher up or on blankets. You can even cover most of the cage with blanket to retain heat during night time.
Before snugglesafe I used two plastic boxes with same floor area but different depth. One was a lot shorter than other so they did slide in together nicely. I've cut out opening in the top box for entrance. And placed hot water bottle in the bottom box, then placed top box with a lid on and stuffed it with hay. It was lovely and warm! And they couldn't get near the water bottle to chew it.