Hi Konrad,
1) Get a cheap USB-serial-TTL dongle from your online electronics discounter of choice. The TTL part is important. (There are plenty of CP210x, FTsomething, and CH340 based designs out there. And they are really cheap.)
I found a project with hardware including gerber files here: https://www.electronics-lab.com/usb-updi-programmer-pcb-for-avr-micrcontrollers/ Maybe that helps a bit :-)
5) Tell AVRdude to use a "serialupdi" progger (because that's what you just hacked together).
Which one is a "serialupdi" progger? In my config I found the following updi named proggers, but which one can be used for that simple usb2-5v-serial-1k prog hardware? Thanks Klaus
