Do you have a bass resonance issue? I'm more inclined to explore other options such as re-running a room optimization software, moving the subwoofer, adding a second subwoofer, or even just moving the crossover frequencies to a lower setting to see if that helps. I'd rather add room dampening or panels if there's an issue that can't be addressed with the tools you already have.
Bass traps are unlike other acoustic treatments in that they need to be membrane-like to work. They're more like hollow boxes than absorptive foam that you can stick on the wall. I jury rigged bass traps in one of my apartments; it was the ugliest thing ever, because each one was about 12 cu ft, and I had 8 of them in one room. It totally worked, but don't do what I did. | 
