You could probably build one out of all metal for not much more if you find the metal at a scrap yard. We do that all of the time for building one off research equipment frames and what not. if you dont have access to a welder you can pay someone to weld it for near nothing. sand blast it and slap some primer on it… or go all out and make it a bolt together metal frame and get it powder coated…
I agree that the hallow pipe will be more rigid (I can do the mechanical engineering beam deflection proof if you want). However, if weight isnt an issue (and it should’nt be) I will still do a solid bar since the repeated weight being dropped on it will eventually fail the hallow pipe and someday it will catestropically fail (bad). The solid bar will just slowly bend / dent (a much more favorable failure mechanism).
When getting a metal rod for the saftey bars, dont just get something super hard. The harder a material is the more brittle it is. You would much rather the bar bend (much like your olympic bar does) then be hard and snap on you.
The pull up bar is bad ass…