I put on the mount and light below OP. The whole rig was less than $100 when I got it, about $70. If you used a 1" scope ring, it would be even less. Its a Laserlyte Tri-Rail and a SteamLight 2AA PolyTac 130 lumen light.
I mounted it on the side, because it interfered with the sling, but my thumb hits it perfect there too.
Its not as nice as the forearm light, etc, but it works and isn't very expensive.