A large mature oak tree in our backyard has hole about 8 by 12 inches that extends at least a foot or two down into the base of the trunk. If it were filled with water it could hold a couple of gallons. This hole is not recent, and other than the hole itself, there's no evidence of decay or other damage.  We've had different advice on dealing with this, from removing the tree (not desirable) to excavating and drying the cavity and then "roofing" it over with sheet metal that would be caulked and nailed to the tree bark.  I've found different advice on the web, but somehow preventing water intrusion seems reasonable.  The problem is how to do this without causing more damage.  My thought is to nail a rubber flap over the hole.  This would keep out most water and still allow inspection from time to time.

What do the experts advise?  Thanks.