Something that is either M-Lok or Picatinny. No keymod.
Carbon fiber is welcome but not necessary.
Believe it or not I still like picatinny quad rails. It is what all the rifles I ever saw during my service had and I think minimizing the number of different parts in the attachment can be beneficial. e.g. device attached to rial vs. device attached to rail and rail attached to forearm.
TED