Thanks for writing.
The PowerSnoot will fit the Canon 600EX RT, but you may also consider the Collapsible Snoot with PowerGrid. The Collapsible model is a little more accommodating to larger flash units like the 600 or Nikon SB910. Either option will work, but some photographers have told us they prefer the collapsible model for that particular flash.
Please let us know if we may be of further assistance.