Keep them warm with appropriate clothing and specially designed heating pads.
Heating Pads for dogs (dog products, beds, varied)
Hound Warmer: 12 volt heated pet bed (dog products, warmer) Akoma dog products
Heated Travel Pads (human products, automotive) Trillium
The travel pad is designed for human's to sit on while driving their car. The plaid fabric cover will easily damage if a dog scratches to nest; please sew a sturdy bag to cover the pad. Dense polar fleece sewn in double thickness can be effective. Plus, later you can remove and wash the sewn cover.
Trillium 12-Volt Heated Travel Pad, sold by -- Amazon, -- Target. Plugs into car outlet or cig lighter. Can set it to heat for 30 minute cycle. Will automatically shut off. Warming cycle will restart when car is shut off and on. Or manually flipping switch.