The product mark was pointing out would work, you might have to trim down the end that starts at the door hinge though. As far as using spray foam, that stuff WILL hold moisture for a long time, it might create some rust issues and if you ever wanted to remove it......nightmare in that small of a space, it could create noise transfer problems to the cabin from the road by linking the fender to the cabin over a large area (that's just speculation on my part). I'd go with the boxes that mark pointed out, mount them the same as steel (pop rivet) then somehow undercoat the entire thing with rubberized undercoating to seal out moisture and dust from the road (the rubber/steel fender bulkhead that's there doesn't create a perfect weathertight enclosure), I'm probably gonna' do the same thing when I get to that point so I'd like to see how you eventually solve this problem, some Pics would be good if you can get them
edit; I'd also do a dry run with the speaker boxes before you do any cutting on the car or boxes just to make sure that they will fit in the space, I've never used them, so the depth of the box is an unknown to me and they might be too deep for the space. Another way to mount them would be from the inside of the car, hold the box up to the spot where you want them, trace it out, shrink that dimension so your hole will be just large enough to insert the box yet still provide a mounting surface for the flange of the boxes, caulk the flange of the speaker box and install. You still might want to pop rivet them in place or you could use self tappers.
