Reinhard,
I'll be interested to see how much snow water equivalent you get. I made a heater for my VP2 cone, and get similar results to what I measure when melting the snow that falls in my 4" CoCoRaHS gauge.
One suggestion is to apply a coating on the surface of your cone to help moisture run down into the hole. I find that with light snow, much of it evaporates on contact with the cone's surface. I apply Rain-X, a windscreen rain repellant product on the cone, which helps a little.
Steve