The Printed Circuit Board and Aluminium Case are the only custom parts you need to order, all other items are off-the-shelf, and you can find them on eBay, Amazon, and Alibaba. Without a set of keycaps, you will spend around 80 USD for all parts. With the optional parts, you will quickly hit 120 USD and still have no keycaps …
Your keyboard will work fine without the following parts. But, if you want to go an extra mile with this project, you should definitely order the Shrink Kit and these items.
With a few modifications on the Circuit Board wiring, you can shrink the Aluminium Case down to 10mm of height. You just need the following additional parts and, depending on your soldering skills, some patience.
The current version of this guide does not include any information how to assemble these parts. Hopefully this will be added in a future update.
If you don’t want to spend the money and time to build an own keyboard, but still want a customizable one with 68 keys, you can order a Magicforce68 keyboard and replace the circuit board with a custom one. The folks at keeb.io have an excellent tutorial for this!