Is it that hard to....

Nixlimited

When I had my STi, the shop I worked with built me a custom rotated mount kit, which then went on to be a production kit. I can tell you it took WAY longer and was substantially more expensive than just getting an OTS kit. You're not going to save any money doing yourself unless you can do the fabrication too, and even then you will probably have a less reliable and more difficult to repair kit. Also, the kit will not work until you have all the parts so buying a part here and part there is no different than just saving money until you can afford a complete kit.