So, Ive been thinking about this too. The one solution I found was https://www.fsplifestyle.com/en/product/TwinsPRO500W.html . The reality is that this is the most redundancy that I need. If Anything else goes it can be replaced more economically than keeping more machines going.Not sure if it would be usefull as a third node for a HA cluster. In case the PSU would die you would loose both the server and the NUC CU and the last remaining node would lock because there wouldn't be a quorum. So a dedicated PSU for the NUC would be recommended in case that server got no redundant PSU so a normal NUC box might be a better choice.