Read a lot of posts on this bracket situation on how to stop leaks, seal, etc. Everyone recommended to NOT fill it with foam since foam will still soak up water over time and get heavy, then removing it would be a chore.
I think that condensation alone occurs,even if sealed somehow....dunno, not an expert there but what everyone was saying.
I thought of maybe filling with ping pong balls since they are water proof and have floation and fill properties to remove most of the air void, so that if it does leak, it is limited weight of water vs the air space in the ping pong balls. That is probably a lame idea also for some reason I am not sure yet.
Interestingly, when I got my boat a couple years ago, I noticed that someone had sealed the drain plugs with the epoxy paint that they used to paint the bracket. I felt the transom was somewhat heavy, so I chipped/scraped/cut the epoxy away and pulled the drain plugs. Lots of water ran out of both holes for quite a while. Man, that was a lot of weight. So someone thought they got it sealed.
So, I pulled the access plates off, dried out over winter best I could with a small fan, sealed up and put an air test to it. Found some small leaks with soap test around the motor bolts. Resealed from inside, leak tested, fixed. Then sealed the access ports and screws with silicone.
Now, when I go out for about a 12 hour day offshore, remove the drain plugs, still get about 1-2 gallons of water. So no idea where this leak is.
It has a solid seal around the transom to bracket. Each bolt on the bracket shows signs of sealing due to some caulk flow out where it compressed with the bolts were tightened up. I know the access plates are very well sealed, yet, still a leak. Will do an other air pressure test this winter and see if I can find where the new leak is.
Bottom line, is I think unless you store the boat in the water, don't seal the drain plugs and remove them when you pull from water. If my boat was stored in the water, I would have to look into the ping pong balls or some other item that would fill the space but not soak up water.