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.
I ran a similar setup for a while that I thought worked well enough. Then I took a shotgun class and found out that the mount wouldn't stay secure through all the rounds that got shot during the course of the first day. So that night I tightened it all down real well only to find out the next day that it was too tight and had bent my magazine tube just enough to seize up my follower.
I spent the rest of the class using a loaner 870, and then I went home and ordered a new tube and a Surefire fore end.
Bottom line is that if you are going to clamp something to your magazine tube, be careful to not over tighten it and plan that you'll have to use some Lock Tite on it as well. I wasn't willing to go that route since I had no guarantee I wouldn't over tighten it again, and I sure didn't want to be using a hair dryer to heat up the Lock Tite every time I wanted to disassemble the gun. YMMV...