The M&P 15-Sport is issued with a non-standard handguard. It lacks a heat shield and is narrower in diameter than a Mil-Spec handguard. I've not attempted to mount a rail to the stock 15-Sport two-piece handguard, so I can't give you many suggestions.
What I can point you to is to a carbine length, two piece, drop in rail system. No replacing the front sight post, delta ring, etc.
Midwest Industries: Tactical Handguards
https://troyind.com/rail-systems-accessories/rails/drop
If you don't want to spend that much $$$, hop on E-bay. The first rail I ever installed on my 15-Sport was an aluminum 2-piece drop in carbine length quad rail. I think I paid about $25 for it. I liked it enough and bought two more for holiday presents. My buddies still have them mounted.