Or cut down the standard FSB, it'll be a great low profile gas block.
Of course that depends on whether you can get the FF tube on. Depends on which one you choose.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together. You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek