Building some more pipes revealed that there is still room for further simplification.
Soldering the nut onto the threaded rod probably isn't the best idea because of annealing the brass. Besides it is an unnecessary step, because I could take a screw instead. Taking a brass M6 x 8mm hexagon screw would fit perfectly. All I had to do is tapering the head and drilling the holes.
This looks promising and easy to make. But there are two problems I need to solve yet. The first is to find a convenient way to taper the head on the drill press. The second is a way to improve the sealing.