Anyone run a Pico inside a pumpkin overnight without it dying?

The issue is almost certainly condensation, not just trapped moisture, because pumpkins are naturally cool and porous. When the screen and the Pico start running, they generate heat. That warm, moist air inside the pumpkin rises and hits the cooler pumpkin walls, causing the humidity to drop enough that it condenses into liquid. Even with the Pico W, which generates very little heat, the screen is the primary heat source. The slime you described is basicly water pooling from that condensation. Regarding your questions about ventilation versus sealing, a small vent hole is much better than sealing the board. Trapped moisture will eventually corrode solder joints and components long before it causes structural rot. You need to break the pressure seal and allow that humidity to escape. A few strategically placed small holes, maybe a single 5mm hole in the lid, pointed upwards and downwards, should provide enough passive airflow without compromising the aesthetic too much. You want a constant, slow exchange of air, not a blast of air.

If you want to take the humidity further, you could look into a small desiccant, like a silica gel packet, but those need to be replaced regularly if they’re exposed to moisture. A better hack might be keeping the display slightly elevated so air circulates underneath, or maybe using a small, whisper-quiet fan to gently push air through the cavity. However, unless you build that fan to be completley sealed and isolated, you're just moving the condensation problem. The simplest fix remains ensuring continuous airflow while keeping the electronics out of direct contact with the pumpkin pulp. As for the screen heat and rot, the heat itself isn't speeding up the rot significantly unless it's an extremely powerful screen, but the moisture is certainly the primary danger to the electronics and the decay of the gourd itself.

The issue isn't just letting air in; it's about managing the thermal gradients. A vent helps, but if you're letting warm, humid air rush out, it's still exchanging heat and moisture too rapidly. When I built a similar display for a client last year, I found that using a thin layer of foam insulation under the board helped significantly. We were dealing with a difference of about 15 degrees Celsius between the internal board temprature and the ambient pumpkin temperature, which drives rapid condensation. Try raising the board slightly and lining the base with somthing non-conductive to break that direct thermal bridge.

I agree that airflow is the biggest factor here. When I built similar displays, I found that the biggest killer wasn't condensation on the sides, but the pulp juice dripping onto the components when the pumpkin settled. I always mounted the Pico to a small riser, like a piece of acrylic with rubber standoffs, just to ensure the bottom of the board was completely clear of any liquid. That little bit of elevation made a huge difference in keeping the actual chips from shorting out.

I had a similar nightmare when I built a weather station inside a sealed wooden chest. Even with a good fan, the internal air just felt saturated. I found that using a small tray lined with activated charcoal sheeting made the real difference. It wasn't about the holes, but actively pulling the moisture out of the immediate enviornment. It made the whole internal atmosphere feel drier and much more stable. Have you considered a small dehumidifier setup inside the enclosure or is the charactrer of the pumpkin making it too damp for that approach?