I would probably be wise to start with something relatively simple.
This old thread of mine, the slip-in serial port connector for the K3 seems pretty simple:
https://www.mobileread.com/forums/sho...06&postcount=1
Especially since I now have all of the proper materials (including what was, 3 years ago, an unusual thickness PCB strock).
A bit on the small side, but nowhere near as funky as this Pyralux overlay I intend to make for the newer models.