I believe that question was already addressed in another thread. From what I understand the FAL flash hider won't work, it's just a little to short. And besides it's a "FLASH HIDER", not a muzzle brake.
As far as a L1A1 flash hider goes they are 5.5 inches overall length but only add 3.330 to the end of the barrel and are 9/16x24 rh thread. If you installed some sort of sleeve in the very end or how ever far inside it to get it to the length you need that might work. Also FAC lists the FSE fake L1Al hider so you could use that or wait till the ban hopefully ends.