You could unscrew the Hose Fittings From the Cooler and make 2 circular copper or Aluminum Compression Discs that would fit into the tapers inlet of the Oil Cooler Nipples and the Hose Fittings would fit over them and seal off the hole.
They do not have to be absolutely circular just so they seal.

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84 300D, 82 Volvo 244Gl Diesel