Alright boys well it looks like the exact diameter is definitely 65mm for the rear midranges if you want them to clip into the mount. About 2.5" exactly.
If you go with 2.25 or 2" I would imagine you could hot glue/silicone then in place easily (as Prop did with his tweeters)
About 1.25" inches in depth -
Hope that helps - I didn't get to get my Memphis midranges in these weekend but hopefully during the week. And now I'm super pumped to do my JL components in the front doors after seeing what semco Mike did - excellent work!