I'm using Ikea Lack as print box (with 100mm raiser). As expected, the hot air accumulate on top, where bottom is relatively cooler. After 10hrs printing the top is ~5'c hotter than bottom.
I made this air circulation pipes/stand which fit standard 70mm fan, to circulate the top/bottom air within the print box, to get a more even temperature.
The pipes come with 50mm / 100mm / 150mm height, one end with mounting plate to mount 70mm fan. You can print "two" and tape them together, so both end will have mounting hole.
The stand is 50mm length. You need "one" placed at the very bottom, so there is space for air to get in/out.
I would recommend you mount the 70mm fan on top, blow air upward. I found this setup caused littlest noise. That's how I'm using (as photo shows).
Nope this is not optimized for aerodynamic nor thermaldynamic :)
I add 80mm fan version, using the same basic design. I'm now using 80mm fan setp: 50mm stand + two 150mm pipes + Noctua 80mm fan on top flow upward.