The only company I recall that I believe manufactured in the USA was secrist kits many years ago.
I am guessing from reading your post, and maybe I’m wrong, but your only looking to have a few kits manufactured, not a couple hundred or more? If your only looking for a handful of kits to be produced I don’t think a factory on its large scale operation will just produce a few. They work off volume contracts usually. I had contacted a Chinese factory years ago and the cost is very high, but the more you produce the better rate you may get.
Def silicone while a different medium would be your best bet if you want small numbers and total control…lots even pour their own kits with a bit of start up investment and some learning or courses…or you can have someone who offers the skill to pour for you for a fee.
I don’t know about now, but many years ago I purchased a kit from BB that was from a German factory, had to of been at least 5 or 6 years ago. So I don’t know if they still produce in Germany, but it is def not a new thing for them if they still are.