There is a guy who make door step plate/sill for the Altima and I asked him whether he can make some for us Mazda3 owner and her said it's possible. I'll get him the measurements and be a guinea pig and let you guys know about the progress. He used to charge $70 for a set of 4 and you can customize the letters...
Here's some pic of my old car:
This is his link: http://www.dintek.on.ca/altima/sidesills/
Here's some pic of my old car:


This is his link: http://www.dintek.on.ca/altima/sidesills/