Business Card Holder with Top

I got the inspiration for this from Dmatsumoto's thingiverse.com/thing:1637 calibration piece. I was having all kinds of trouble with filled-in pieces, but my Cupcake would make beautiful single strand walls.
So I drew a single strand walled box to fit a set of business cards and I added a bottom. My version has square corners because I ran into trouble in Blender when working with the rounded corners, I think it was the way I added the bottom. (I could not open the exported stl file in Replic