dude you use caulk on the inside of the box, for the sub you could use a gasket or some foam i think and sometimes you really Don't have to use anything and just screw it in to place.
polyfill only makes your box think theres more internal volumn then there actually is. So if you made your box to exact specs you should not need polyfill.
Correct me if I'm wrong, but doesn't the fumes emmited from any type of siliconized caulking eat away at any foam parts of the sub? I was under the impression that silicone or siliconized caulking must be completely cured before even placing sub into box. True?