Visit your local welding shop, schedule 80 black iron pipe.was commonly used in the good ol days. Think thats what you are after. I have 40ish feet of 1/4" in my junk pile. All you need is a pipe thread die.or get them to make up what you need.
Depending on volume requirements,3/8" steel brake line could suit your needs too. A little small for bigger machines.
Myself, i would weld a section of tubing to boom and run hydraulic hose in it.
Edit:scratch the black iron pipe. Sched 80 doesent even come close to pressure requirement. But a welding shop will have the proper material