The 309A from Electro-Voice is a professional-quality suspension shock mount. The mount is constructed from metal and has a swiveling basket for precise mic positioning. The mount is designed to support the RE20, RE27N/D and RE320 microphones. Because these mics have internal shock mounts as well, the 309A provides an additional level of protection against unwanted vibrations.
This shock mount seems like the standard option for folks who buy a RE20. Makes sense - they're designed to work together. Before buying this, though, I'd urge you to take a look at how much the rubber replacement bands cost - they're about nine bucks apiece and you will need to replace them every year or so. I went to third party bands made out of a different rubbery material that did indeed have better durability but those, too broke after about 18 months. I ended up simply getting a different shock mount for my RE20 after that. The 309A is otherwise fine. It's a little unwieldy and large but it gets the job done.
These are not cheap but make it a lot easier to comfortably position this mic in front of a client from a boom.