Shops like Maplin often sell battery heated gloves at reasonable prices. Not sure what voltage they operate on but can't be too hard to adapt.
The Maplin ones use three AA batteries in each glove. They sit in a small plastic box with slide switch, loose in a zipped pocket on the back of each glove. The box connects to the glove lead though a standard DC plug, so easily adapted to an external supply, omitting the battery boxes.
They heat to 40 degrees C peak and the three AA batteries typically last a half hour. The heating element is on the palm just below the first finger joints, so probably not robust enough for work that requires very high grip pressures.
Not sure if they are still available, I can't find them using their search facility but that means little since it's not a good one.
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