When I bought the kit it didn't include a diagram. But with how tight the space is I think you would struggle hard to make a one off any cheaper as there would probably be a lot of trial and error.
I personally think for the quality of the hdi kits and what's included its a steal.
You gotta remember it's nit just a core with a few bits of pipe. It also includes all your joiners and clamps, plus the required brackets for installing the after market thermo you will need to install. And in comparison to all the jap kits it's sooo cheap.
The other thing is the his kit as proven flow rates and design. You might save $200 getting something made and then have it being a peice of sh#t. And then still have to fork out the extra cash to fix it or make another.
But that's just my 2 cents.