Acoustic treatment for problem corners.

My old space had exactly the same problem: only one corner of the room was a simple corner, the other 3 having a door, a closet, and a bookshelf.
 
Two answers. First, you can still get some benefit from corner traps even if they don't cover the whole height of the room from floor to ceiling. Just fill in as much you can.  Second, corners aren't the only place to trap bass. The junction between walls and ceiling are useful, too. You might want to consider putting traps up there, all around the room.

As much of it as you can cover! Getting enough bass trapping in a small room is difficult. Some would even say impossible.
 
However, in my case I experienced a small but noticeable improvement by covering the wall-ceiling juncture across just one wall - the one behind the mix position. That was as far as I got with that room before relocating to the garage, which was much easier to treat.

Might be worth a try, but my guess is that it won't have much effect on low frequencies. Acoustical foam is different from packing or mattress foam in that they treat it with an acid that eats away some of the material, turning the air bubbles into a matrix of tiny tunnels that open to the outside world. 
 
Even acoustical foam suffers from limitations due to its low mass. You need mass to absorb sound, and you need thickness to absorb low frequencies. So think thick and dense. Now, if you could suspend a full-size mattress up there, that would soak up a lot of energy! :)