How to Change `collection_check_boxes` into X Boxes?

With this...

  <%= f.label "Level 1:" %>&nbsp;
  <%= f.collection_check_boxes :missed, Habit::ONEMISSED, :downcase, :to_s %>

I get this (assuming the user clicked on the first checkbox):

[ √ ]"X"  [  ]"X"  [  ]"X"

What would I need instead so that when the User clicks on a checkbox it shows an X instead of an arrow?

EX:

[ X ]  [  ]  [  ]

The latter case looks cleaner. Thanks in advance for your help =]