I was also thinking about your idea for the housing for the front speakers. As I was a stereo installer previously I think if you were to put the mid bass speakers in a small housing such as a headlamp or similiar you will lose lower end frequency response from the speaker. The speakers were designed to play in a door cavity or something of similar size just like a sub is designed to play in a certain sized box.
A sealed enclosure for them is however a great idea. The size of such could be determined by using a computer program with the speaker parameters. Or you could ask the Focal distributer in Australia. he's a great bloke and i'm sure would be more than happy to help if you were using one of his speakers. I guess the appropriate size would be somewhat like 3 to 6 litres, probably the larger the better.
If you were to use the three way system the mid range speakers would not need to be housed in such a large enclosure at all as the frequency range they play is much higher.

Hey JB
met you today at breakfast with Lowey.
just had to join up to say you've done brilliant work with this thing. I cant wait to see it finished.