For building your own Mega65, refer to https://c65gs.blogspot.com/ or to Chapter 12 of the Mega 65 Book, available at https://files.mega65.org/manuals-upload/mega65-book.pdf
The model is intended to be used with M3 threaded inserts.
Please consider sharing your Makes here and/or share your experience with this model over at https://www.forum64.de/index.php?thread/132281-self-built-mega65/
The author hasn't provided the model origin yet.