Electronics: Re-Used Cell Batteries

Here's a couple of old cell phones of mine. The batteries (3.7v 900mah Li-On) are in excellent condition, and the phones can be used as charge regulators- so why not use them as flight packs for a small model aircraft or robot? Have any of you done this? I'm wondering how I could build a light battery holder with "precise enough" terminals to contact the battery tabs? All thoughts are appreciated.