The best way to make it safer....leave it in the garage.
Seriously, that's just the plain facts of these cars. I think airbags would be nearly impossible, due to the complex electronics, and the fact that they are designed to work with the specific vehicle they were originally installed in.
I think door beams are a good idea, and most race cars are set up like that. It will certainly help, but as you pointed out, safety and Cobra shouldn't be used in the same sentence.
The best thing I would do is high back seats, properly built car, wear your belts, and most importantly...drive with your head.
That accident was terribly tragic. But it is an inherent risk in driving a Cobra that you may be hurt or killed if it is wrecked hard.
Steve