Here's what I did:
Gun is laminated wood stock Remington 870 Express 12 gauge with 4 shot magazine tube.
1 1/2" Picatinny rail from Choate at riflestock.com that I screwed to the right side of the forearm. (about $10 plus shipping) Choate also sells a short plastic forend with predrilled holes and rail included for $31. MidwayUSA also sells Magpul picatinny rails that are cheaper than Choate's but do not include screws.
I mounted a Streamlight TLR-1 on my shotgun from midwayusa for $99. It will illuminate targets up to 75 yards and will really brighten up a room. I originally had a Streamlight TLR-3 that Midway sells for $84.99 but it is now on a handgun. The TLR-3 will only illuminate targets up to 40 yds away but will work fine for you and is smaller and lighter.
The nice thing about this system is I can easily actuate the light w/o taking my hand off the forend yet it is out of the way when operating the action. And if I want to take the gun hunting or trap shooting it takes less than 30 sec to take the light off.
I hope this helps.
P.S. Sorry I was going to attach some pics of my setup but they were taking forever so I gave up.