I already had the MTM discussion with this person and that is sort of a dead end discussion.
I agree too though that buying vs building is likely the way to go on this one. Can a DIY be done in this case? Sure, but it will be far more difficult, and likely more expensive.
To armed, do you have experience building a speaker at all? If you've never built one, I'm going to say you might want to try a kit speaker first, which has the crossover already designed for the drivers that are included, and see if you can find one that will fit the space you have.