I have no idea how well this fits a QSC CP8, because I bought them to protect my Alto TX308s which are about the same size as the QSCs, just 1cm taller, although the sloped edge for using them as monitors is opposite handed. I could not find tailored covers for the Altos anywhere. They fit pretty snuggly front and back all the same and will definitely protect my speakers in transit in my car.
I do have other Thomann covers for other speakers and have always found them to be of decent quality and they do the job they are intended for.