I order one yard of fabric that matched the fabric of our custom table runners, table squares, and ring bearers pillows (Michael Miller; Dandy Damask in Black). I then dug out my handy dandy hot glue gun, a little stitch witchery, a ruler, and a marking pencil (all things I keep around due to my random crafting urges).
I just measured the height and circumference of the bottom piece of the hat box and cut a piece of fabric to accomodate. I left myself about 2 inches of extra fabric on all sides but you could definitely get away with just an inch. Then I used my stitch witchery to create a hem on all sides, leaving a nice creased finished edge all the way around (sorry for the blurry picture).
I then started at one end and hot glued a thin bead between the fabric and the hat box. I noticed my fabric was exactly the right size (about 3/16" too tall) so I was careful to keep the fabric even across the bottom edge, so the box would still sit evenly. Finally I made a pretty bow out of some white satin ribbon with my Bowdabra (I told you I was a craft junky) and hot glued that centered on the top of the hat box. Viola! Super easy lemon peasy... took approximately 30 min after spray painting.
I still have not decided if I am going to cut a hole somewhere in the box for guests to slip their cards into or just set the top off to the side... What do you think?