A very simple business card holder. No supports needed. No brim needed if your bed adhesion is good, otherwise a brim with some post-processing should work fine. 

There are two models. I found the basic Business Card Holder.stl worked for most cards, but when I made this, the person who requested it found their business card was slightly too wide. This is why this model also includes the cleverly named Business Card Holder Longer.stl. 

If you have some calipers handy and want to be sure to reduce waste, the exact measurements of Business Card Holder.stl can be found in the attached Business Card Holder Drawing.pdf. The only difference between this model and the Business Card Holder Longer.stl model, is the latter is exactly 6mm wider to accommodate slightly wider cards.