Actually you don't need to have the sensor on the opposite side, since the light in the tissue scatters in all directions (with the fluctuating blood pressure in the finger altering the amount of light absorption). I would think it's even better to have the sensor on the same side as the led, or at least have sensor/led on left/right sides of the finger as opposite to top/bottom sides, since you don't have to worry about the finger nail getting in the way. It is after all an extra thick layer the photons have to penetrate.
I'm thinking about butchering this:
http://www.ellos.se/DetailPages/DetailPage.aspx?productId=129782 (Price is approx. ?20)