Thanks to everyone who gave input on what to use for the borders of the little kettle piece. Your input really got me thinking, especially because there was no unanimous choice. So I went back to the stash and chose these.

The fabric on the bottom was once a tablecloth that was once used on the boat, where the copper kettle lives. (And before that it was intended as a sarong.)
The borders are now securely attached, and today while stewarding at the London Quilters exhibition (which you can see, quilt by quilt,
here) I started quilting them.
This piece is evolving step by step; I woke up one morning with the idea, and got on with it, and am enjoying seeing it develop.
Some people advocate doing all the planning before starting any of the hands-on stuff. In her article in the Summer 2008 issue of the SAQA Journal, Brett Barker advocates doing multiple sketches: "If you can manage to do three or four sketches, you will find that your idea will refine itself without any conscious thought at all." When her "creative" brain has come up with a final sketch, her "thinking" brain can make fabric choices: "Doing preliminary work through sketches allows me the freedom to create without stress - the hard part is done. I can now sit back, relax, and watch my idea realize itself in thread and fabric."
Sounds like a good way of working - or is it a counsel of perfection? Elsewhere I've heard people express horror that some people don't draw anything out before starting a quilt, don't use sketchbooks, don't spend "enough" time planning. Is that always so bad? Is it a Golden Rule to spend 50% of your time doing the planning? Consider the oriental method of working: visualise the result and then do the work; if the outcome seems good, put it away for a few months, then reconsider. Yet another approach is to "act like an artist": do something, and respond to the outcome.
Which of these is (or are) the most appropriate for something as labour-intensive, and as drawn out, timewise, as a quilt?
There are probably as many ways of doing the creative work for a quilt as there are artists making quilts. But let's hope there aren't "design police" ready to lay down the law on this - there are already too many "quilt police" busy inspecting the finished items.