F1ach: I think the biggest problem would be designing the covers, eg. I have 2 diff galaxy phones, i bought covers in different places.
One cover has the retainer strap go over the back of the phone when closed, the other goes over the front of the phone when closed, the one that goes over the back is better, when you unhook it, its at the bottom of the flap that drops down.
The other one is attached to the top and irritates my ear when talking.
The reason for mentioning this is that, apart from getting the small things right in the design, you also have a myriad of different phone shapes and sizes to contend with and market.
Maybe make something that fits inside the battery compartment or maybe something that will activate when you whistle like the keyring things you can get.
Either way, I would buy two if they fit that sort of criteria.
Lucky an engineering is experimenting with using the phone battery to power it but unfortunately as you said there are different shapes of phones, however we will try to make it fit as many phones as possible especially the most popular ones.