I had an extra piece of PRI rail laying around, and noticed the inside of a USGI synthetic stock I use for field purposes was already "inletted" to allow the inside metal to clear the gas cylinder, etc. up front.
The rail is positioned to allow sliding the weak hand forward, and activate the light with the thumb while maintaining a normal grip on the forearm.
This shows a comparison of the M1A Scout with how my Zombie rifle is set up. A 90 lumen Streamlight TL-3 LED is held by a Botach mount, but any light, including a 500 lumen Surefire M4 can be used depending on need(hunting Lions?!)