Somebody somewhere has got to have a 5/8 24 FS. Kurt just finished my upper and put a nice SS FS on there for me, although it did take about 2 or 3 months. I haven't seen it except in pics, but it looks real nice.
Find yourself a Vortex FH or an ArmaLite factory FS.
And if you're currently running an ArmaLite brake, that's good in the interim.