I made this little model for mounting the RPi3 on my printer. I love when you can see circuits and everything of a board, and printing the entire case for it would have hidden the Pi. So I bought an acrylic case on Amazon (https://www.amazon.it/gp/product/B06VWD8M6X/ref=ppx_yo_dt_b_asin_title_o02__o00_s00?ie=UTF8&psc=1) and printed the last layer of it with the support for the Ender 3. Like this, I can mount a fan on the Pi with the case I bought, and mount it on the printer.
The model has 2 main holes, one for the sd slot and one for the heatsink provided with the case, and many other holes for better airflow under the Raspberry and for using less PLA.