The only bulbs I've found that work in tight enclosures without overheating and failing are a design that's probably 90% aluminum heatsink by weight and over 50

The only bulbs I've found that work in tight enclosures without overheating and failing are a design that's probably 90% aluminum heatsink by weight and over 50% by volume, with a relatively small dome diffuser on top. I couldn't find a US distributor, so bought a box direct from the manufacturer on Alibaba. They were $20+ per bulb even from the factory, so not cheap. Beyond the crazy cooling, they have a thermal shutoff feature to prevent failure if they get too hot. Generally it's better to find an appropriately breathable fixture for LED bulbs, or even better, ditch retrofit bulbs and get something with a separate DC power supply.