Here's the thing: the manifolds I've built are not pretty and they're gobbed together from old bits of VW downpipe. That's not to say they don't work, but it's a very different proposition hacking something together from scrap for yourself, vs building something for someone else, for which you would have to ask proper money and get third party liability.
At the end of the day, to build one you need some 10mm plate, a good hole saw, a number of VW downpipes, a grinder with a few skinny cutting discs and a reasonable welder.
Ideally you would also need a second engine which you can bolt the plate to while you work up the manifold, which will stop the plate distorting when you weld stuff to it.
A six-branch would be a rather different proposition as the characteristics required would probably be quite different to a 4-branch. I would need to speak to the technical department before starting on that.
Tip up at my place with the car, required materials and a couple of hundred quid and I could probably be persuaded, especially if you can make tea without thinking about putting your c0ck in it.