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The Eastern Spotted Newt (red eft) is supposed to hibernate over the winter. But ours are in a 5 gallon terrarium. In an unheated outbuilding, the whole thing would freeze solid. They wouldn't be able to burrow deep enough to avoid freezing.

Has anyone out there ever hibernated pet red efts? If so, how do you do keep them cold, yet not let them freeze? Are there any pre-made products out there that are known to work?
Safely and reliably?

Does your house get colder in winter? That is probably enough. You can enhance this by putting the terrarium near a cold window, or in the cold part of the house, or putting a frozen bottle of water in it each evening. Another possibility is the garage if it does not freeze there. But don't forget they have to eat even when it's cold, they do not really hibernate.

Hibernation Considerations:
1. Start with healthy, plump newts. Since the animals will be 'resting' for anywhere from one to three months, depending on species, you only want to hibernate newts with the health and fat reserves necessary to make it through their 'winter'.
2. Allow the animal to empty its stomach and intestine (the 'gut'). Retained faeces and food in the digestive tract could putrefy in the body, which can cause death. 'Cleaning' should be done at normal temperature before cooling starts, as the metabolism then slows down preventing defecation. (At 25ºC/77ºF, a newt takes an average of 33 hours to empty its stomach and intestines; at 5ºC/41ºF it will take about 220 hours (yes, 9 days!).) (Side Note: Some newts will remain active through cold periods – though not through a Candain/or north USA winter!. The Red-spotted Newt (Notophthalmus viridescens viridescens) of Ontario, Canada is (anecdotally) known to be an aggressive hunter/breeder while the ponds are still covered in ice in spring!
3. Allow three weeks for gradual cooling of the newts and reduction of their photo period.
4. Provide clean, airy hibernation quarters with a suitable deep substrate into which the animal can burrow. Substrates have included damp sphagnum moss (squeezed until non-chlorinated water no longer drips from it) and paper towel. The trick is to find something that won't rot. Please keep in mind that some plastic containers will 'weep' chemicals when filled with damp material for any length of time.
5. Place the hibernation container in a dry, frost-free situation and keep a routine check on the content's moisture; a maximum/minimum thermometer is useful. 5ºC/41ºF seems to be an adequate 'resting' temperature, though some species of Red Spotted Newts from northern climates will still be active at that temperature, and may in fact eat during winter if food is available. (Though it must be noted, winter-dining newts only consume about  of the food size that they do in summer; they eat smaller creatures than usual and less of them.)
6. Consult a reputable book on suitable temperatures and conditions for your species.
7. Do Not Hibernate an animal which shows any sign of illness or has recently been ill or injured.
8. Do Not Hibernate underweight animals or those who have not fed well lately. A low-weighted female newt that successfully hibernates will not usually develop eggs; and if she does, her chance of being selected for mating by the size-fixated males is doubtful.

References:
The main part of the above is text from: Davies, B&D. "Frogs and Friends". Aquarist & Pondkeeper Vol 60, No 8, Nov 1995. p.71.
Additional points added from:
A. Massey. "Notes on the Reproductive Ecology of Red-spotted Newts Notophthalmus viridescens" Journal of Herpetology Vol 27, No 1 pp 106-107, 1990
KM Ries & ED Bellis. "Spring Food Habits of the Red-spotted Newt of Pennsylvania". Herpetologica Vol 22 #2, pp 152-155, 1965?
Suping Jiang & DL Claussen, "The Effects of Temperature on Food Passage Time through the Digestive Tract in Notophthalmus viridescens." Journal of Herpetology Vol 27, No 4 pp 414-419, 1993
PA Verrell. "Male Mate Choice for Large, Fecund Females in the Red-spotted Newt Notophthalmus viridescens". Herpetologica Vol 41 #4, pp 382-386, December 1985