The first question about any product is size, and it is the one most often answered too late (sample note).
A card mock-up takes an afternoon. It is ugly, it is flat in places and it falls over, but you can hold it over a mug, put it in a bag or stand it on a desk. On the Pour kettle, the card model showed in ten minutes that a 0.8-litre kettle was far too big for one cup.
Make three sizes, keep one, and throw the rest away. Nothing later in the project is ever that cheap again.