For this part of the project, you can decide in which direction you go. In the end we want to install a Raspberry Pi in an old Leitz Prado Retro LED-Beamer. The best way to do so is to mount it inside the cage below. The Raspberry Pi fits rather well.
buying is difficult as they are not many adapters that fit (special pin layout) and most are too long
if you want to build one, you have to deal with very precise soldering
decide how to get audio out
you can use the analog audio output of the Rasperry Pi - then you need to do even more precise soldering as your adapter
you can use some Raspberry Pi hats for sound (I don't know if they interfere with the pins needed for temperature monitoring)
you can use Bluetooth audio (didn't get that to work)
or you can use an USB audio device (used this method with a Bose sound station which turned out to be a good decision)
check your Raspi skills
if you are good at running Raspberry Pis headless, you can put everything together from the beginning
if you are not very experienced with running Raspis headless, you should configure everything on the flly with a keyboard attached and a larger display. You won't be able to work with the small 2.0" display at 320x240 resolution.
Download Raspbian Lite (Buster) and write it to a decent SD card (with Etcher, for example).
Then go to the config directory (in Windows it is shown as an accessible drive) and create a file named wpa_supplicant.conf. Edit this file and add your WiFi credentials to connect to your WiFi.
country=US # Your 2-digit country code
Add a file names ssh in the directory to enable ssh access.
Mount the DS18B20 (optional)
If you came here from the Retro LED-Beamer Project, you want to do this step first. It's about mounting the DS18B20 to the heat spreader/LED unit to measure the LED temperature.
Don't buy the transistor alone but rather buy the complete probe consisting of a metal probe and a short cable. You will need a 4,7k-resistor, too. Follow this instructions to solder the resistor to a the correct cables and connect the pins to the Raspberry Pi: https://raspberrypi.stackexchange.com/a/48360
Now get a 6mm drill and drill a hole just through the heat spreader. The hole must be at least as deep as the probe's length.
Put some thermal grease onto the probe and slide it into the heat spreader as deep as you can. You might need to slice the heat shrink tubing a little.