I read about using a part from an air-cooled VW engine and adapting it for that purpose. By the time I cut it down, it was a lot of bits (2 pcs telescoping sleeve, nut, O-rings) to try to fit in a space of maybe 2-1/2 inches. Maybe less. Not sure where my sketch is. I couldn't make it work.
The end result: I used the rubber tube. A bit of effort to degrease everything, but straightforward. A bit of RTV on each end and I wrestled it in. No more leaking and pretty satisfying.