I have a Giant Trance. I have derailleur hanger with a Goat Link to accommodate a large cassette in the rear. Where can I get a custom made derailer hanger made to eliminate the flex with the goat link and hanger?
With enough work on your end to design the part and also the willingness to swallow the expense, it would be trivial to get this made by a machine shop, i.e. one of the various online outfits where you send in a drawing and they send you a part.
The second level to the question is that manufacturers aren't typically very public about their hanger dimensions. In other words, they don't just give you what you need to model them up quickly. I suspect the third party producers reverse engineer them with fairly proprietary techniques and tooling. There's a game there where getting it most of the way is probably fairly easy, but really getting it to a dialed fit could be tricky. The straight lines and angles are easy, but basically everything on a hanger that's got a curve or a rounded edge will need to have that radius measured faithfully to feed into your model. You're not going to be modeling out the GoatLink itself - all that matters is the target point-to-point dimension and angle relative to the axle, plus getting the b-knuckle interface modeled correctly. In other words, it would probably make more sense to design the hanger in a way that "connects the dots" with the shortest possible distance, basically making an overgrown Shimano DM hanger.
While a cool idea, it may be worth pointing out that Goat Link and related setups (anything that simply drops down the RD) have a tendency to give compromised shifting at the best of times due to the nature of what they're doing. The pulley is much further away from the cog than intended throughout most of the range because the slant of the RD parallelogram is always going to be a poor match with the cassettes in question. They also add flex, so your premise here is not incorrect, but that's far from the only element that negatively affects performance.
When the GoatLink came out, there were few or no other options for doing what they do. Now a better solution to the flagship application, making Shimano mountain 10 run 1x on a wide-range cassette, is probably just put on an RD-M5120-SGS. In both cost and performance, that will likely be far better than designing your own extra-drop hanger, and if you want it will also let you clear a 46t.