You have 3 options for a barrel extension,
1 The jerry-rigged method of converting a bren bolt and carrier to work with a modified ar15 extension. (Make sure to use a left handed extension as the bren bolt rotates the opposite of an ar15 bolt)
2. Killing a barrel by hacking off breach end just behind the barrel fixation, then boring it out and thread the inside there by making an extension out of your previous barrel. Do note here that the factory barrels are very hard so you will need to use proper cutting procedures for cutting hardened materials.
3 make your own extension from scratch. You can have a machine shop EDM cut the out raceways or always just machine them on a mill with a rotary head accepting some additional radius or finishing the lug cuts with hand tools. Back boring, threading, adding a chamfer on the jnside of the lugs, cutting feed ramp grooves. heat treating and then finally truing the lug faces.
Any of the above will work