I guess it depends on what you want. Are you going to keep it Drop-in or convert to Free Float. I have tried a lot of different hand guard setups for Windham SRCs (got 2 and 1 MPC).
Here are a few "modular" Drop-In options
1.) Magpul MOE hand guard. Not the best, but can have rails added 2 it. I would not run any optics or sights on it, its hard to keep them zeroed.
2.) Command Arms M4S1. A little different looking. Two QD sling points. One lower rail can be added and 2 smaller side rails. No top rail option.
3.) Midwest Industries SS Drop-In. Super light for metal. The top rail is not modular. To me this one feels the most solid.
And here are a few non-modular ones to consider aswell
MI Gen2 Drop-in quad
Strike industries MITCH
AB Arms Hand Guard (railed and non railed top versions)
As of Right now one of my Windham SRCs has the VLTOR CASV-EL Free Float on it (also a modular system)
And my other SRC had the MI Gen2 Drop-In quad (from above)
As far as Free Float systems the VLTOR is nice. I have also tried a few others. off the top of my head
Midwest Industries Gen2 Extended Carbine FF
Or they have a modular SS Free Float
Troy Charlie Battle Rail
Troy Delta Battle Rail (the modular version of Charlie)
So look over all these and go from there. These range for $40 to $250 in price so nothing super high end (like DD, Knights, or LaRue). All work well with SRC and consider that with covers even a "cheese grater" can feel good. Also think about the Troy (Charlie and Delta) and the VLTOR CASV-EL convert you to Free Float with only the removal of the front sling loop. So if you ever wanted to go back to Drop-in, your Delta ring and cap would still be there. Hope this has helped you some.