Finished. It was really easy. I used 4mm needles as they were already lying around and some fuzzy yarn.
Cast on 3 stitches and knit one row.
Increase 1 stitch at the beginning of each row until the desired length is reached.
My cone was the inside of a machine knitting cone. I have a few left that I kept as 'they would probably come in handy sometime'. They have now as ...
I have a few other fancy balls of yarn left. These came from the CS. I've started on the white and this time I'm using 5mm needles to knit it up even faster.
PS The triangle knitted is an equilateral triangle. I shall have to experiment to see if I can do an isosceles triangle. Maybe I shouldn't increase every row for this.