This works for me. The cobra plantlet at the end of the stolon should have some roots established before you consider cutting it off the end of the stolon. I would leave approx 5cm - 10cm of stolon (2 - 4 inches) on the plantlet. With the remaining stolon, I would cut it off as close as you can to the main plant and cut it up into approx 7 cm (3 inch) lengths and place them ontop of live sphagnum moss. I would then place more sphagnum moss over the stolons until you can't see them. You could chop up the sphagum moss you place over the stolons.
As the winters here in Melbourne, Australia are not as cold as where you are. I don't consider placing a cover over them. I treat the pot of stolons the same as a cobra plant.
Care for the removed planlet like you do your parent cobra and hopefully new cobra plants should be produced from many of the stolons. Some stolons may rot, while others should produce new plants. This method is a lot quicker than growing cobras from seeds.
I have found if you don't remove the plantlets and one or more are left attached to the main plant (via the stolon), these plantlets tend to grow taller (larger) than those removed. The parent cobra contributes to the plantlet growth.