Can I hang a countertop microwave oven under a cabinet with custom hardware?
Yes, you could build this yourself. It wouldn't be easy for someone without metal fabrication tools and knowledge on hand, but it could certainly be done. You'd want to try and procure the templates, which often have location dimensions printed on them.
It's worth mentioning that third-party manufacturers have made it available for barely half the price you quoted. Unless you have a lot of time and ambition, that seems the prudent route. Part #JXA019K.