Interview "Thank You" etiquette (see details)

I had an all day interview on Tuesday and I think it went really well. I met with 10 different people throughout the day and this was a second interview. They paid me for the day and took me out to lunch. They also said that they would probably have a decision by today but may have to wait for the background checks to go through, which could take up to 5 days.

After the first interview, I sent a quick thank you email to my main contact there. I am wondering what I should do for the second interview thank you. I want to send a hand written card but I am wondering if I should purchase a Thank You card or if I should send a plain one? Also, should I address it to my main contact + the wonderful staff at (insert companies name)?

What would you write?