Finished! A very quick project using two strands of DK and a 5.5mm hook and trebles into spaces between trebles. The base was 66 stitches in a circle to start with. I found a large wooden toggle with conveniently large holes so I could sew it on with a large tapestry needle. The straps are just simple lengths of dc's attached in the right places at the back. I'm sure they will stretch a little but that's ok because Ella reliably informs me that it's nerdy to wear it too high up. Who knew?