Many fanless pre-built and configurable PCs are available from our friends over at SilentPC.com (not sponsored, not affiliated) including this waterproof and dustproof option.
Also, check out this classy Comet Lake x Streacom combo...
... or this Xeon x Turemetal server. According to SilentPC: "Each of our systems go through a rigorous burn-in and testing process ensuring that each system is quiet and stable before it ships. We only build with the quietest, most reliable, components available. Every system is custom assembled in Battle Ground, Washington. If you contact sales or support, your call or email will be answered by a technician. No long phone trees, queues or ticketing systems to wade through."
