expanding foam should do the trick.
have you ever taken out a tree with foam in it? i did one about 2 weeks ago and i had some extra time so i bisected the cavity (first time I got a chance with foam to do so) it held alot of moisture and seemed to promote rot even more, we use to fill them with foam as well... up until about 2 weeks ago haha, it was a good little learning experience
I was conversing with a friend of mine with more credentials then i can list, (he's forgot more then most of us would ever hope to learn in his many years doing so and studied under shigo himself in his earlier day) and he says that for the most part he just cover it with a steal mesh to keep animals out and leaves the cavity open to alow for proper air flow, a drain whole could be added if the situation requires it. but every situation is different and really its the situation that depicts what you should do.
I'm not critisising in anyway this is just a little food for thought, it changed my outlook on it