First of all, CONGRATS!! YOU PASSED!!

I think here in Canada people have a large N posted in the back window to mean New Driver, but also a lot of people make a sign with the words New Driver and put that in the back window. I hate it when people are going slow on busy roads without signalling what they are doing. What, are they drunk? But if they have a new driver sign, I am very patient, and I think most other people understand. You could make a sign, even if no one else in your area has done it that you have seen.