One thing you might find helpful is to use standoffs and shaft collars in “unconventional” ways to support straight runs of C-channel mating at angles to each other, etc. The black plastic shaft collars will also work in ways kinda similar to the steel ones you see in this thread:
You can use the threaded inserts to join them together or, as in the case of the black plastic collars, regular screws going through the collars and into the standoffs.
