Tripod mount/2x converter

I recently bought a Tokina80-400 which I thought came with a tripod mounting ring, but it didn't. Perhaps its only the Mk II that has it. The lens is a bit heavy to be supported by the camera tripod mount on my fairly light tripod. It tends to droop.

Does anyone have any idea where I might find one? Also, I am thinking of getting a 2x converter, and wondering which would work best with this lens, or is it a stupid idea? I saw some Bower ones in New York, which seemed to work with my Sigma 10-20, but it does not get very good reviews, especially for its autofocus capabilities. One shark in a camera shop on 5th Avenue tried to sell me one for $400, an incredibly brazen piece of attempted rip-off artistry since they go for about $75!! I won't be going there again!

Rgds,
Jon