Traveller does have a purpose built point defence turret - the sandcaster. I am not entirely sure on the need to design an alternative but you can do as you wish in your own game.
Fitting a missile system for anti-missile work should be less problem than doing it for guns. You can put smaller missiles into the launch tubes used by larger ones, or you can replace the missile's warhead with a smaller missile or point defence weapon. Or you could just use a large nuke and detonate it somewhere near the incoming. You also need new electronics, but you needed that for the guns too.
One shouldn't overlook electronic jamming and decoys. There's a lot of stuff there. Take a look a system like Nulka. It's a rocket that hovers around a ship, pretending to be a ship. (Although I would assume it moves slowly away from the ship ...)
My personal opinion on Traveller computers is that they're not that good, and they don't like automated systems. Computer control of air/rafts isn't standard on most industrialised planets, and like Star Wars, they probably have a person behind each turret in combat. They have anti-hijack software but it never works: how many scenarios are based around that?
You do get a nice combat DM in classic High Guard, so the computers clearly do something, but it's not as much as we would expect today.