I tried a saddle mount and reflex sight on a Remington 11-87.
I ended up scrapping the idea.
The saddle mount sucks donkey balls.
It's too high, moves, and over time it marks up the receiver.
The best mount that I've found, is the Burris SpeedBead. The best sight, is the Trijicon RMR RM02. It works with the SpeedBead mount, by using the Trijicon RM37 adapter plate.
Unfortunately, the SpeedBead mount for the 870/1100/11-87 is the worst of all the SpeedBead mounts.
It's not Burris fault, so much as how the stock is designed to fit with the receiver on those guns.
You can make it work, if you have some technical savvy and are mechanically inclined.
But, you also can't be afraid to do some cutting and JB welding on the stock.