In case anyone found this information useful, I am only using 2 QRX100 at the moment and didn't need the 2 extra spaces. I ordered Digikey part #3490K-ND which are 1.5" round aluminum 6-32 threaded standoffs, which fit the 2 units near perfectly (could be ground down about 1/8 or 1/16 of an inch if desired). For my situation, I can appreciate the slight space savings. A package of 10 is $12.34.