A cowl designed to keep your neck warm and look good even if you open your jacket a bit.
Use two strands of sport or DK weight yarn held together, or a bulky/super bulky yarn - the two strands will make a denser fabric.

The tip is knit flat, the turtle neck is knit in the round on one long circular (120 cm) using magic loop, but you can substitute with any other method to work in the round. For a circular without magic loop, 40 cm is a good length.

I’ve used two strands of a discontinued sport/light DK yarn held together and 5.5 mm needles to achieve a firm, dense fabric that wouldn’t let too much wind blow through, as I use this cowl mainly while biking.
If you use two strands, you will need about 330 meters of your sport or DK weight yarn.
Of course you can make it with one strand of a thicker weight yarn (aran or bulky) - as long as you get gauge and like the fabric, it’s up to you.
Of this thicker yarn, you will need about 165 meters.