Not a fancy fix but it will work.
You can make an Insight Technologies M3 light mount on the fore end.
Purchase a long Weaver scope base and attach to the bottom front of the fore end, or side if you prefer. I used a combination of a screw and bedding compound to attach mine. So far this mount has held very well and you can very easily slide the M3 or similar light on and off of the base.
Takes up virtually no room on the fore end, costs very little to put on, and lets you take the light off when not needed. By doing the work carefully you can even set it up to shine the light beam to the point of aim for your shotgun. Something which my Surefire $170 fore end light (On a Rem 870 12 ga) will not do.
Rob