You could probably just screw them right into the panel but .. how they would sound would be a different thing maybe? Im not too sure, I've never seen it done personally.
If you want to make some enclosures for them I think that would be your best bet as your making more room for them to sound like theyre supposed to.
Like i said to you on MSN, think about mine.
I've got pretty much a 6.5x13 (Technically more(Not including gap between each speaker)), since I've got 2 6.5s beside each other on each door, so yeah its possible.