[brak]

I use a "swan neck" fitting. I had mine custom made, it really makes things easy and no water intrusion is possible.

Looks like this:

The tube projects for about 1" downward from the flange, which makes it sit pretty strongly in the deck.

 

[brak]

It's not obvious looking at the posted pictures but the edge on top of the tube is well rounded as you can see on this illustration.

This is the design I had on my boat before switching to the swan neck. It worked ok until water level around the mast got high enough (mostly when a big wave would come to visit, but also when pooling from rain and such). When that happened - it would leak a bit. (There was also a leak inside wire jacket which made it look worse than it is, but I found that one only after replacing the fitting ).

 

[brak]

Perhaps putting neoprene foam (1/2") variety at the top of the cover would cover that. pressure of the foam against the wires when the cover is bolted down would compress around and at least slow down the water ingress, in this set-up....

Actually, compression of wires was one of a reasons I replaced it. It did help to keep the water out but it was also breaking and chafing the wires at the point where they were pressed against the edge. It could probably be remedied with a smoother edge design too, but really swan neck seems like a more elegant solution to me.