If I want to use use hot water from my coffee urn on Shabbat (as I understand it, the urn is a kli rishon) to make instant coffee, I pour it into a separate cup and pour from there into the cup that I intend to drink from. While there might be all sorts of halachic intricacies which would change this process, I'd like to start this hypothetical with those parameters.
Does the kli sheini into which I pour the hot water have to be separate from the kli rishon? Does it have to have a certain size or shape?
Could I attach a small receptacle with a secondary spigot to the spigot on the urn so that I can consider that "way station" to be a kli sheini (as it is not heated by the urn) and I can open the secondary spigot and pour from there into my coffee cup? Would the intermediary have to be of a certain size, or shape? Would I have to wait a period before emptying it into my coffee cup?
