I'm in agreement with the folks suggesting a crab tattoo -- probably the best symbol of Karkat besides 69, which has other connotations. A wrist tattoo of his speaker crab watch would be funny but that's kind of excessive... I would recommend getting a tattoo of the little black crab we see on Eridan's beach, page 2066. It looks like Hussie put the MSPA reader face on it, which is fun, and since I imagine you guys have limited time for the group tattoo session it would be nice and simple