I'd rather just build a custom case for an extended battery, and attach it with a dc coupler and use sugru to get it to line up perfectly. Cheaper, easier, better battery life, and better ergonomics than the ownuser grip. Started to mess around on sketchup and autocad working on a design, hope I can get around to building something, but it is a long ways away. 3d printer would be prime right now. Rapid prototyping, and I could make all sorts of other useful things