I've got an M&P-15OR also. I bought a Yankee Hill folding front sight (YHM-9627)
that is designed for mounting on a tactical forend, shorter than the one made for the gas block. Because of the higher gas block this is the route to go. I'm sure there are others that make folding front sights for the top of a forend.
Here's a link:
http://www.yankeehillmachine.com/store/sights.html