You can make a 6.5" fit if you get a little creative. Do you have 4 grills laying around, or maybe head to a local install shop and see if they have 4 that you can get for cheap and make them work.
there was a thread over in diy forums on how to make some. Might wanna search cant remember. You could use the material like on my crossovers too if u could find some i bet.
Where did u buy your dmm robin? I got to get one soon, so i can adjust my eq to the likings and reset gains. What etst tone did you use? -3 db?