Do it right. The spray foam would would well. Though, in the end I would really recommend a sealed enclosure for the speakers. You'll get more low and mid-range out of them and a flatter frequency response. I really hate those cheap foam speaker baffles that people buy, as they are usually too small of a space. I think the best way to do it is use the Bose front speaker boxes and modify them for your new speakers (provided you have enough clearance for them to fit in the box).
I was very close to purchasing some Focals a couple of months ago, and instead went with some Dayton Audio raw drivers only because I'm running in an active crossover setup. I retrofitted them into the Bose speaker boxes and couldn't be happier with the results.
