can this be used as a diffuser on the wall behind the speakers?
Yes it can and wood is the best material for diffusers. This will mostly diffuse upper and high frequencies - since it has a shallow depth, bandwidth will be narrow, but better than nothing...and cheap.
Call GIK Acoustics and ask them for help in selecting a product.  You may decide on something completely different to meet your needs.
I like your thinking ,cuz I was thinking the same thing

Im sure there are many NON-audio products out there that would satisfy as as diffuser, like at World Markets above.

Although, I would not suggest to go behind Safeway, grab some wooden pallets, then come home and nail them to my wall. :)

The GIKs are popular, they usually always have a presence at the Audio trade shows, like CAS last month. They seem to have a pretty good marketing sense: eg, I believe that they brought a bunch of extra GIK products to CAS and let the audio manufacturer rooms borrow the GIK products, rather than the manufacturers having to bring them themselves to the show. 
Well, it is either that, or the audio reps try to get creative with blinds and blankets to try to manage room acoustics. 

But back to the wood diffuser: Im sure there are many DIY options, or cheaper options - eg, get a carpentry student or sculpture student to make you one to specs, etc 

If you're interested in making your own, here's a thread to one I designed.  Performs very well and almost ridiculously easy to build if you're somewhere that sells US dimensioned lumber --

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