Nick - The weight of the box should make it so it doesn't keep sliding up and down though it depends on the box. I used a belt cut through the back of the box to hold it to my chest - the friction from this should also help keep it open.

Unfortunately its quite hard to describe how to make it but hopefully the video and pictures help. Email me some pics of your creation - would love to get them on imafish - contact me through http://imafish.co.uk/contact and I will send you an address to email stuff too - look forward to seeing what you do!