Gunshots has a really nice hitch on his Classic. Send him a PM and find out where he had it made or got it. They might do one for the R3T.
Be apprised of one thing, however. Triumph will immediately void your warranty if they observe a trailer hitch on the Rocket. The key word is 'Observe'. If you take the bike in for any service you need to remove the hitch. If you don't, it will be noted on the workorder and become part of the bikes history.