9" high would be too short for any waveguide speaker I can think of unless you put the waveguide right between two 6.5" woofers. But that's not a good idea. Maybe there's a chance of that with the smaller 8" waveguide, I'm not really sure.
Even 8" woofers need a box that's 9.5", so it would be tough.
About the smallest I could see working would be ~10" with 6 Dayton RS75T 3" mids below the SEOS-12 and a pair of 8" midbasses on the sides. It would obviously be a 3-way. The RS75T's are only 2.3" tall and you can fit 6 across easily. You could do 4 with a higher crossover point but you would lose some sensitivity.
When trying to maintain a timbre match across the fronts IMO it is as important to match the mid's as it is to match the highs. I would probably use the same drivers on the L/R but with the midbass's above and below instead of on the sides.
The real catch here is that no crossover is designed for this. Going active DSP would get very expensive with 9 amp channels.
One other possibilitiy would be a SEOS and a single pro 8" woofer to one side. This isn't something I've done and I haven't thought about it much so I don't really know what warts it might show. Directivity is obviously going to be a bit funky but I'm not sure what that translates to as far as actual performance.