Easy way to make custom headers? What ??

Yep that clamping system looks nice! The cheap way to do it is to use painters masking tape.
Have done some headers and unless you are seriously into classracing or prostock or something at that level just build something that fits in the right size wile trying to get your primarylenghts somewhat right,not worth loosing any sleep over if its perfect or not.

Just think atleast twice before cutting,keep the sections true as you cut them and tack everything togheter as you fit it.
Fully weld when you have the layout the way you want it unless its impossible to get to the weld due to tight spaces.
I prefer tacking all tubes together then break them lose from the flange and collector for final welding of all welds on the tubes after all four tubes per side are done before welding to collector and flange.
And here is one for the rookies that alot seems to have a hard time understanding,the smallest radius is not always the best choice in tight spaces.
In places where two tubes are close to each other and "spooning" ,use different radiuses for a much nicer look,outside tube can use a larger radius to follow the inside tube much cleaner looking.
Always use the biggest radius you can use,its better for flow.
if you end up with a few "scrap" pieces of straight tubing and bends in the begining,dont worry you will find a usefull spot for them later on and they are certainly handy to have to be able to get an idea about if more tubes will fit next to the one you are presently working on.
Try to get your cuts as true as possible and make all parts fit each other as good as possible or you will hate yourself for it when you start welding..
Most mandrelbent tubing will be slightly deformed,dont worry alitle preasure from the right dirrections and a few light taps with the tolerance adjusment tool(hammer) and it will pretty much perfect if you just have nice and true cuts.

I could probably keep on typing on this subject forever,its not rocketscience but just getting the right feel for it and figuring out the litle things that makes a whole lot of difference goes along way.