Couple of points to preface my answer:
1: Do you have the money to do it again?
2: If you've got the time to band-aid, do it right the first time.
3: Are you in a position to entertain making the room worse?
4: You'd be surprised what miss-application of small space acoustics can do to a larger space. (even a 500 seat auditorium is considered a small space, but it's much larger than a home theatre or home studio)
5: How do you know you can't afford an acoustic consultant?
That all said, proper acoustical treatments, be they diffusion, absorption or both, requires proper design to be most affective. In addition to the physical dimensions of the space, finish treatments, etc., actual acoustic measurements (sound recording or sound measurement devices) need to be utilized. Simply placing 1/3rd or 1/4 cuts of Sonotubes may not do much, if anything, at the frequencies requiring attention.
So, if you're up for a learning experience for the group, and are willing to be patient, we can work on an acoustical design to take care of both flutter echoes in the room as well as the assumed required LF correction.
I read much of the same home made home studio acoustic threads you do, and unfortunately, there is little evidence of applied acoustics and applied mathematics behind the postings. "It looks cool and the room sounds better (different)" seems to be the main themes sounding on those threads.
Churches, auditoriums and even gymnatoria are different beasts all together. My mentor (I studied acoustics from him for about 8 years, he's had me on my own for the past 2-3) is well known for his large cathedral work. He does the rare small room, which he defines as anything below 1500 seats, but unfortunately, small rooms can cost more per seat to correct.
Let me know if you're interested in the "Acoustics Self-Help" study.