He's going to have one hell of a job cutting a theard with a die as the wrench will not be able to rotate within 40mm. but you can buy die nuts which is a die within an hexagon and you use a spanner to cut the threads, but if you do use a die use a cutting oil.
Does the original poster want the thread part added to the existing thread, rather than cutting more thread on the one already there?
You can buy straight threaded bar in metre lengths with various diameters and threads; these can be carefully bent to make your own U-shaped piece.
I have done this for custom antenna brackets for various unusual satellite dishes as well as homebrew ham radio microwave antennas.