jmo
I too have been going down this same route.
I too am going to chop the tail and noticed that the TT frames are better for this and I prefer the frames fixing points to MM.
However as far as I know the MM cases come with their own mounting system, which if you were to use the TT mounting system, which personally I don't like! you would have to remove.
So we really need someone to try MM cases with TT frames (has anyone got TT frames yet?
Maybe Paul at MM would help, but I fear he isnt going to promote using any one elses frames?

I had a good look at the MM cases and do consider them way above anyone elses cases (even at the price) I too want 31Litres.