I replace my Ignition Lock Tumbler with a Meyle one (they are not supposed to be any good but I did it anyway). It has a similar plastic key head.

I took a knife and sliced the key on the side, removed the metal part and filled the space with JB Weld Epoxy and re-assembled it claming it together till the epoxy cured.

That was in Aug, 2017 and no issues with the Key Head or the Tumbler for that matter.

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84 300D, 82 Volvo 244Gl Diesel