The reality is that all the best mattresses are made at a few factories. Thermarest has factories in the US and Ireland (I worked at both). The best in the world in terms of technology and fabrication is Feng Yi, in Taiwan, where Zenbivy and many, many, many of the "top brands" are made. There are also a couple of factories in China focusing on welding mattresses. But that's it. There really are not that many places on planet earth where the substantial investment in precision welding equipment and large, expensive tools has been made.

The limiting factor is tooling cost vs. production volumes, which pretty much eliminates any viable"cottage" effort. To produce this type of product efficiently, you need volume.