I’ve been using old ssd cases (aluminium ones), which would otherwise end up in trash. Drill four holes, use a screw to create threads in those holes. Lay some thermal tape, and mount the pi. I haven’t seen it rise above 60degs — plus it looks alright in my opinion. Few pictures here [1].
I got rid of the ports because I don’t need them (headless) and so they mostly fit entirely. I’ll make some sorta cover though.
Edit: Forgot to mention that these cases are strong. It fell a few times and nothing visibly bad. I added Pimoroni shim in second iteration because I did want to keep hdmi there. Also left the Ethernet leds for indicator lights.