There are several ways to do accomplish the same task as the frame generator, one of the many useful design accelerators built within Inventor. You can definitely make a wire frame 3D/2D sketch, set of planes, axes, etc... The thing you will learn about CAD is there is almost always more than one way to complete a task, it is finding the most efficient method that becomes the challenge.
If I were tackling this problem, depending on what I was trying to accomplish (design intent), I would likely create multiple configurations (
SolidWorks term), Inventor calls it
iParts, of the rail and assemble using planes and other rails. This allows the assembly to be somewhat dynamic and easy to adjust. I think if I was doing a lot of weird (non-orthogonal) angles and planes I would stick to the frame generator, it makes trimming cleaning up the ends/corners of the members much easier. Again, I would try multiple ways and figure out how everything works, and then know what method to use as you tackle a problem.
Note, I have not used the iParts, iAssembly, or iLogic part of Inventor so I don't know much about it. However, I am a huge fan of SolidWorks' configurations (our team uses SolidWorks and the video posted above) and PTC Creo's family tables (the software I use at work) all the time. These really save a good amount of time on common parts (lengths of rail members, hole location relative to end of rail, etc.). I found this video of iParts and thought it was pretty informative:
https://www.youtube.com/watch?v=oNLCPaOqjaA. You can also look up videos and applications of SolidWorks configurations for more ideas of how to implement this feature (just know it is a different software package).
-Tim