Sound diffusers

Well the CD rack is a diffuser but foam eggshell is an absorber. So which is it?

Anything like your staggered CD solution will work to scatter reflected sound. Anything soft and porous will work to absorb, ie convert acoustic energy to heat. 

If you weren't so cheap I would recommend Owens Corning 703 as that is extremely effective, readily available, and easy to work with. 

Its actually good however that you are cheap. Because the best way of saving money is spending time. Time spent listening. Trying different things. Like for starters you drape some fabric over your CD rack. Let what you hear be your guide. If you hear a difference then that will direct you to better solutions. If you hear no difference well then so much the better as then you don't need to do anything.

Interesting that you mention fabric. My wife is a fabric artist and has made me large thick pieces which I drape behind the speakers for absorption. I need to put wool or synthetics behind those. I’m trying to tame the first point reflections at this time. The “skyline” boards are heavy and beyond me. The studios use hard panels of pointed cones in a fractal array. I wish I was skilled enough the make the panels described on arqen.com. Perfect and scientifically based but beyond my skills at this point. 

I’m cheap and am not willing to pay big bucks for commercial studio pieces.

I guess it comes down to what you consider to be "big bucks".  I paid $125 for a single 24x48 inch diffiuser/absorber from GIK that looks and works fantastic.  It's engineered to perform< I suspect, much better than some randomly organized CDs.   The sound impact is amazing in my space with these products.

True. And two would be all I need for my two channel set up. The speakers are 30” from the side walls and I only need two panels at the “mirror” points. Thanks.