I did what you described. I filled the cavity with foam and covered with vinylester glass (it resists water
penetration), creating much shallower and smaller drain box where both manual and electrical bilge
hoses end. My main reasons for doing so, beside one you mentioned:
- original cavity is very deep and hard to see whether there is any water
or foreign object
- if there is water/dirt/etc., it is hard to clean/get out. I have long arms, but not long enough to reach where I need to
- the aft portion of keel cavity is so narrow that it is very difficult to clean
Time will tell whether it was wise thing to do...
I keep my bilges dust-dry and clean. If I see water there, I need to find where it came from.