I don't know if you've already done this, but you might want to do a search for Stephen LaBerge's original research into the best physical activity to track to identify entry into REM. I could be wrong (it's been a very long time since I heard him discuss it) but I think he experimented with many different things, possibly including heartbeat and galvanic change, and found that the most reliable activity to track was eye motion.

I think making your own induction device is an excellent idea, but don't be afraid to use -- and maybe improve upon -- the devices of the past.

Good luck!