This sounds like fitted insulation for the equipment being installed, similar to pieces I've seen to keep faucets from freezing. I'd simply use a literal (and appropriately descriptive) translation in this case and call it a "foam shell" or "foam cup". There's an example in the first link. I suppose you could also refer to it as "foam half-shell" (see the second link) - it's not terribly common, but it describes matching halves of the insulation nicely.