I could not find any good photos of how people have solved cooling for the ACA 1221ec when overclocked so I am posting mine.
https://imgur.com/vNN6N0F3D Printed cover and a Raspberry PI heat sink. The heatsink height prevents a standard cover from closing so the new 3D printed cover has a hole large enough to allow the heatsink to stick through (that also improves cooling). However heatsink is small enough that it is still flush with the Amiga and does not stick out.
So far heatsink stays cool enough to hold your finger over easily even after playing Gloom
I am also using the ACAGov software which switches the CPU running speed depending on current processor load. This seems to work surprisingly well.