My husband is a teacher in an after school maker program and his students often make repair parts for items needed around the campus. Vacuum cleaner parts, replacement covers for lights, etc. Lower tech than what your talking about, but similar idea. As this is a class, they haven't had to turn it into a business model. They are also using a printer that can print durable materials.
This experience has DH and I considering a similar business model to your own. We are looking to purchase some personal equipment (3D printers, routers, laser cutters) to start an on demand design business. i think he is planning to market his designs to hobbyists to start. For instance we've helped make custom replacement parts for a guy who restores old radios. Seems like hobbyist are willing to pay to get the custom part. We may also start an Etsy shop for some decor designs. We have several different product "lines" that seem possible.
Like you we are still trying to figure out how to make it a side business. One thing we will do is find a few items that seem to be more widely marketable and create a standard design that we can quickly produce/keep limited stock on hand. (Like your window part -I bet there is a market somewhere for that) Then also offer a custom design service for an hourly price.