This is probably the densest one-stop shop for handwiring resources. It has links to some of the other tools I used, and QMK is the open source firmware that runs many of them (I used a fork that enables a better graphical key mapper), though there are others that can be easier to set up.
On Youtube, there's a kid who has made a cottage industry out of these. Search for Joe Scotto.
In general, searching for "hand wired keyboard" will bring up a lot of resources. Then, if you're so inclined, learn how to do a basic PCB in KiCAD and you start closing in on a device that works every bit as well as a commercial product.