Has anybody come across the use of non pumped refrigerant passive cooling for Intel CPUs ?
Heat pipes and the like come to mind.

The size and shape of the enclosure will have to be adapted to the process.

The ideal to acheive is maximum reliability by using the least number of moving parts (CPU fans are small and by experience fail first - PSU/box ones can be doubled for redundancy) for a piece of transmitting equipment I'm involved with on a non-profit basis.

The use of a liquid<> vapour cycle in a heat pipe could solve the problem: liquid falls to cool hot area of cycle (near CPU) vapour travels away to cooler area and condenses back to fall onto CPU area. For solar collectors alcohols seem a favourite.