I have one of the forends with the built in light from CanAm, I will swap this from the 18.5 mag to the 8.5 mag when it shows up. Let you know how it works.
Mesa Tactical makes a mount for the 870 that I think would work well for the short 8.5. Just google Mesa Tactical Side Mount Picatinny Rail for Remington 870
Wouldn't happen to have a picture of your gun with it installed?I used one of these for my 12 inch Grizzly mag... it mounts to the magazine plug, and does not need any barrel to clamp onto:
http://media.midwayusa.com/productimages/880x660/primary/754/754094.jpg




























