I'm a fan of the compact version, but I realize some people may want to make their own full size card.  I also realize that these are likely to get built in small quantities and often hand assembled, so I created a full-size hand-solderable version.

The back side is blank with only the antenna (under soldermask), the programming header, and a hole for the LED to shine through.  You can order the design from OSH Park, or get the design files on GitHub.

With the design files, you can add your own customization.  The antenna will be mostly invisible behind soldermask, unless you want to show it off and opt for the clear soldermask "After Dark" option at OSH Park.

You could also customize the card with a label.  Business-card size labels are readily available and then you can create your own full color design.  The label should not affect the antenna, and the LED should shine through if you don't color it too darkly.  Be sure to include some indication where they need to tap.

The header footprint in this design staggers the pins to allow for a friction fit for programming.  I'll post a picture of the design when it gets back and let you know how the staggered pins are working.