I'm not an expert on these boxes, but I have worked with the foam. Although I can't provide a nice, clean "Yes" or "No" answer, a couple of things come to mind:
- That expanding urethane foam is really hard to control, although it does stick to almost anything. It would probably be best to put in too little and add a second area after re-measuring.
- Although the foam develops a skin, will the surface be enough to withstand the constant battering of the sound waves, or will it eventually degrade? If it would, maybe a layer of glass and resin would toughen it.
- Did you allow for the volume of the speaker?