I have used the Hi Vi RT2C in a center channel that I designed and built, The BG neo 3 and 8 are used by some. Various speaker suppliers have a ribbon that seem to look like an older HiVi RT2, PE sells it as the PT2. On the high end there is the Ravens and their clones.
My center channel I used 2 Audax HM170COs which are carbon fiber cone drivers.
What are you tring to design? Is this for HT or music? What drivers are you going to use, how much power, etc