I have an old Mini 14 flash hider that might be adaptable, The thrwaded section slip fits up into it. I eastimate it would add 2 7/8 inches to your barrel that way, the exterior base is the exact same extior diameter as an HK Flash hider. If you are good at welding or have a freind who can you could rebate the sleeve portion and weld the front to the flashhider that came on that rifle, that would give you the length you need. Close the bolt on your rifle and drop a rod down the barrel and get an exact measurement to the bolt face so you know exactly what you need.
Let me know if you want to try it. (PM if yes)