The old DOS program ModPlay came with a file called which detailed several designs for DACs that attached to a parallel port. I was pleasantly surprised to find that ModPlay has its own web page: https://awe.com/mark/dev/modplay.html. The first download link on that page (https://awe.com/mark/bin/mp219b.zip) is a .zip file that contains HARDWARE.DOC. That file was designed to be displayed on DOS. In order for the diagrams to show up correctly you'll need a text editor that can be configured to display text in the DOS codepage (CP437). Notepad++ can do this via Encoding | Character sets | Western European | OEM-US. If anybody has instructions for other programs / operating systems, please feel free to jump in below.

The HARDWARE.DOC file contains designs for 2 different mono devices that use DAC chips, one mono device that uses an R-2R ladder, and the "Stereo-on-1" which produces stereo using only a single parallel port.

I built a Stereo-on-1 back in the day. I had to send away to England to get the required chips from Maplin. It didn't work properly when I assembled it because I didn't understand how the pins on the chips were numbered. They're numbered counterclockwise, starting from the pin with the dot but for some reason I had thought they ran down the left side and then ran down the right side. Years later I revisited this, figured out what I had done wrong, and fixed it. It still worked the last time I checked a couple of years ago. It produces quite decent sound.