haha, piece of cake in the Miata. Although mine isn't a good example.
I would say you should do fine for your first few events without seats and harnesses. Especially if you are only planning to do 1-3 events per year. So that leaves you with the standard or single diagonal. Remember a roll bar is going to take away from the car in a convertible or a regular roofed car anyway. I personally don't trust the roof of a car to keep my head safe if I roll over at any speed higher than 60 or 80 mph.
As for the safety during daily driving, well you are required to run padding on any part that can come into contact with the driver or passenger. Mostly, I would run SFI padding on the roll bar and then get some foam style padding and zip tie that over it for the street. The SFI padding is for helmets, so its pretty hard. With that said, its softer than steel and it has saved people before:
http://revlimiter.net/miata/bonk.htmlI didn't have any roll humps in the miata to get in the way and there was some interior panels that I had to modify to fit my roll bar, but it was well worth it in chassis rigidity and safety.
Do it, the fun is well worth it. And if its that concerning, get a beater to daily. probably cheaper to keep up with than the s2k anyway. And you can drive the s2k for any hotties
