Home Improvement Questions? Ask a Handyman for Answers
Unfortunately you cannot fill the cavity in most circumstances, but I would have to see your particular installation to provide a more accurate answer.
If you can attach a bunch of photo's some taken from a distance so I can get an over view and some close ups of the 'clean out' area, from various angles and very well lit, I can take a look.
Use the paper clip icon at the right end of the tool bar above to attach photo's
We can go from there.