I have a funnel somewhere for this purpose, its got a rubber sleeve on its neck and a valve to open and cut flow, Very useful for bleeding FWD V6 engines on toyota/nissan/honda etc. The funnel sits in the narrow opening of the radiator cap hole and the rubber takes up the sealing duties.

I bought it a long time ago, dont remember what it was actually called.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)