I enjoy geocaching, and sometimes use the iPhone app when caching in the UK. The problem is that it's rather hard on power and so I have a Mophie Juice Pack. Once that's attached, the fabric cosy no longer fits, and there are none available to buy which cover both the phone and the external battery. Fortunately, I have the technology, in the form of crochet hooks and yarn, and last night I made myself one.
This is my attempt to write down what I did. It's sufficiently easy that I did most of the work in the pub. The trickiest part is the shaping to allow a smooth fit over the rounded lower part of the phone+battery combo. That part is right at the the beginning, though!
Use a 2.5mm crochet hook and Wendy Happy yarn - it takes a fraction of a ball, and leftovers of any 4 ply yarn will do. This pattern uses UK terminology - Americans should substitute doubles for every treble!
1 tr into 4th chain from hook. 1 tr into each remaining chain. Mark all the "corners". 15 sts.
1st round: Ch3. Do not skip a stitch, but tr into 1st st. 13. tr. 2 tr into last st. 17 sts.
Do not turn! 3 tr into the side of the tr. 2 tr into bottom of last tr. 2 tr into next tr. 11 tr. 2 tr into each of next 2 tr. 3 tr into side of first tr. ss to close round. 42 sts.
2nd round: Ch. 3. 1 tr into each of next 20 sts. 2 tr into next tr. 1 tr into each of next 5 tr. 2 tr in next tr. 1 tr into each of next 6 tr. 2 tr into next tr. 1 tr 1 tr into each of next 5 tr. 2 tr into next tr. 1 tr into next 3 tr. ss. 46 sts.
3rd and subsequent rounds: Ch. 3. 1 tr into each tr. ss.
Repeat this round until case is desired length. Fasten off.