This thing includes a bracket that can be glued to the back of any little sign. I designed several road signs in an associated thing, http://www.thingiverse.com/thing:298565, but any similarly sized medallion, business card, or key tag you like can substitute. The bracket slides over the top of the post.

Glue the flat back of the bracket to your sign, closed end facing up. Then just lower the open end of the bracket over the vertical post.

Using my printer (a Printrbot Simple) as configured by me - mostly defaults - the post prints best with 3 shells and only 15% fill rather than as a solid object (which can result in a very oozey, ugly deformed vertical part). I used the same parameters for the bracket. No sanding is required for the pole in the bracket with these choices using my printer. I hope it prints well for you, too!