There are a few possible solutions that may work for the Karma.
You can use 3M tape to attach a plate, as seen on this 997:
Here's a post from the forums over at Teamspeed:
Clean the bumper
Shape the plate to match the bumper
Place double sided 3M tape on the plate
Expose the other adhesive side of the tape
Place onto the bumper
Swift Motorsport makes a well-executed motorized plate system as well. I'm thinking this method may be best. Only $150 for the setup.