I spent some time bread boarding popular circuits I found with my ninja like google-foo. Unhappy with the performance, I opted for a dedicated IC [FDC1004]. This IC is both source-able, and gives predictable and repeatable results. I went through three hardware revisions before settling upon "this" one. Changes included the power management circuit, UI layout and antenna placement.

Battery life was also another major concern, I didn't want to use rechargeable batteries (sorry environment), and I wanted relatively long runtime.

I did want an enclosure, but I really liked the way the bare board looked.