Wanted to give my PC case better airflow but couldn't find any designs I liked so I made something simple. This is the first model I've made so far and I'm pleased with the result. Lowered my Idle CPU temps by 5-10 degrees Celcius with an AIO.

Cut with a Demel and sanded the edges until it was a snug fit then epoxied the inside for extra stability and to make sure it wouldn't rattle.

To get a snug fit, trace the outside of the print then mark 3mm to the inside. I cut just inside of those lines then sanded the edges until it snapped in.

Hopefully will work for a variety of cases.