I came across this company doing CNC enclosures made out of solid copper billet ... look ideal for smaller SUTs, perhaps even larger components if they could be scaled up ... reasonably priced too at least at this small size (80mm x 55 x 27):

https://www.ebay.co.uk/itm/P-CNC-Cop...pHqoTdZcEYOBkw

U.S. based though .. I wonder if such a thing is available in the UK/EU? Presumably being CNC they could also drill holes for RCA sockets etc.