I would like this controller to collapse into something you can put in your pocket. At first I entertained the idea of using some sort of elastic material to both hold the phone in place and tension the two halves together. The structural integrity of this solution seems like it would be poor.
Another option would be to custom print-to-fit a specific phone model, using the phone as the structural component and an elastic material to hold the sides of the controller to the phone. There are a lot of phone models.
The option I settled on was to build the controller around a rail system. This system could be 3D printed or sourced out. One of the sources I have considered and will probably go with is igus.com and their linear guide system. Quite pricey, but for prototyping purposes (and for that expensive smooth-glide feeling) I feel like it is the best option for now. This system will expand to accomodate a phone or tablet that is 150mm in one dimension, and collapse to about 180mm total length, 80mm device size. Not exactly skinny jeans pocket size, but smaller and it allows for multiple device sizes.