i always find "theoretical work" questionable ...
The hard work was getting the simulations to a stage where they reflected real life. It's easy to simulate what's happening in crap rooms. Simulating wonky (good sounding) rooms, was the culmination of nearly a decade's work by 2 separate research bodies. We used to check each others simulations to see if our alternative methods gave the same answer.
In the 80's, 3 major loudspeaker manufacturers in the UK built special listening rooms at great expense. All 3 got them wrong cos they used parallel walls. Don't ask me how I know this.
> Obviously, my room is not symmetrical, so L/R symmetry is out of the question
I did say "try". You could move the setup nearer the door but I understand if that's not possible.
Have you got a bad flutter echo in one direction? Which one?
If it's the Window/Closet direction, are things better when the closet is open? See my comment about open bookshelves (with books). They are similar to your "boxes" but are dual purpose.
In a room that size, a few cushions / bean bags on the floor are going to be much more effective than anything you can sensible put on the ceiling.
About the only place, I'd put a formal panel would be directly behind the speakers and that would only be cos there is no space for a bookshelf. Hang it tilted.