The solution I use is real high tech - a hot water bottle!
The hot water bottle is in a cosy. I put a towel over the EPs, the bottle on top, and then a blanket over the lot. The EPs warm up, and the heater strap I put onto the EP at the focuser doesn't need to labour much at all to maintain some heat in the eyepiece.
I've been doing this for several years now. I have never seen any effects of having warm eyepieces in the image in case you are wondering, with many, many hours spent sketching, hence observing, this way. But I am using an open truss dob. I have a 2l thermos of hot water with me. This is plenty for sub zero temperatures that we typically encounter here in Oz. This gives me 1 litre to start with, and enough for a change of water around midnight.
Any 12V solution will really chew through power. There is a lot of bulk that needs heating in any eyepiece case. I considered some type of 12V solution first up too, but the power requirements were ridiculous and ultimately not a practical observing solution. The hot water bottle is just easier. A few of my observing buddies have started using hot water bottles this way too. No foggy eyepieces, and many productive hours
Alex.