I also have a few Pi computers in 2 layer acrylic cases with no sides and 40mm fans. The ones I use also happen to match the width of a 2.5" SSD/HDD, so I can replace the top cover with a drive
if I want. Cheap, versatile, and adequate protection with easy access to GPIO.
SSD mounted on top of case with short standoffs, Noctua NF-A4x10 5V fan, and a CP2102 micro-USB UART (Pi3 in picture is now a Pi4).
Paid $3.90 to $6.99 for most of them, but recently got one with a smaller 30mm fan for $2.99 (all from Amazon with free Prime shipping). I also added knurled brass thumb nuts for feet.
Very easy to change the internal spacing with longer or additional standoffs, or by simply placing the fan on top, unless you have an SSD up there like I do. The linked picture with the SSD replacing the top cover and fan has a Pimoroni Fan-SHIM
For that fanless PoE HAT in the OP the fan on top of the case would not be as effective (it would cool the PoE HAT, but not the Pi4), so you'd either have to rely on the open sided case allowing heat to radiate away, or get additional cooling some other way (Fan-SHIM might work, but it would not allow the PoE HAT to fully seat on the GPIO header). Side fan or passive heatsinks are other options.
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?