I really like the Burns clamps (I forget who they buy them from).
They have a machined version and rolled sheet metal. The machined have a nice lip to locate the parts and I think it makes things just a little easier.
The clamp part also has a snap-acting latch so you don't have to take the nut off all the way a really nice feature.
The Burns clamp has a nice quality slotted and silver plated nut that won't gall when hot. I work in Aerospace and there are a thousand different styles of clamps. The Burns clamp is pretty close to the quality we use on our aircraft valve installations. Of coarse the A/C stuff is stupid expensive!
Also, if you are really into weight savings the rolled sheet metal flange they sell is really nice. In aircraft we use metal seals to keep leaks to a minimum. If you really need a low leak joint you can machine a small groove in the flange for a metal seal. They are also quite expensive. If you want to try aircraft style flanges shop around for parts that meet AS1895 standard that will get you started.
Scott