the driver kind of depends on your host. is it a single battery host? if so you need a boost driver. is it a multi batt host, then possible a linear or buck, or use spacers. what lens are you using? in other words you don't give enough info for anybody to actually help you
I'd recommend a flex drive microboost, or a x-boost. check out dtr's site in the pro shop, or click the bottom links in any one of the threads he's posted in.
There are round X-Boost™, but it comes in 2 sizes and you have to figure out which one you need. C6 I
think is 16mm, but I'm not 100% on that, maybe somebody else can chime in. The round ones go out of
stock frequently, so you can also get what is called a contact board and solder a square driver to that.
The thing about the X-Boost™ is that they NEED a heatsink on the driver chip itself or else it will smoke.