Heat the second hand with a heat gun then press the hot plastic onto the metal spindle. Obviously, you'll want some sort of insulation between you and the hot plastic so you don't burn your fingers - use a piece of wood or similar to apply even pressure. You may want to drill a hole just larger than the size of the spindle through your wood. This will allow you to see exactly where you're pressing and allow the wood to push past the top of the spindle so you can move the plastic to the desired height on the spindle.
You may want to drill a hole in the plastic, a bit smaller than the diameter of the spindle, as a starter. That way, when you push, you're only pushing a bit of plastic out of the way, not trying to make the hole in the fist place.
As the plastic deforms around the metal, then cools, it should shape itself exactly to the spindle and key into any key ways, splines or other shapes on the output shaft and hold itself quite nicely. If necessary, add a small drop of epoxy on top of the second hand to hold it to the shaft after the plastic has cooled.