Your DDR Wieger STG-940 clone looks good, I have collected German parts for four years to build one (SLOW mode) The best deal on a US flash hider is Notherngunparts. You may have to call and see if they make the 1/2"x28 nut.
The second option for the nut is CNCWARRIOR. (make sure you select the proper thread pitch)
Here is a link to notherngunparts:
AR-15 / M-16 A-1 New Birdcage Style Slotted Flash Hider
Here is a link to CNCWARRIOR :
Muzzle Jam Nut