Other things to consider are the
306 sleeve from grateful vaper -
The DCA from Goodsense - and a regular old 306 to 510 drip tip adapter
like this one from Avid.
Also to remember, 306's vary. When you buy a 306 drip tip or sleeve or adapter, it's good to try to find out if it will actually work with whatever 306's you are using. The grateful vaper sleeve and the 306 to 510 from avid definitely fit Cisco 306's - If you are using other 306's, then emailing the vendor about any drip tip, sleeve, or adapter before you buy is a good idea.