If the end of the flue is a single walled piece of copper tubing projecting past the boiler end, could you drill four small holes in the projecting flue from the sides, 2 each side, and drill and tap mating holes in the sides of the elbow, then fit screws through both flue tube end and elbow to hold them together? . As mentioned, any gaps could be closed with fireclay before fitting the screws.
If the end of the flue is double walled with a water space between the walls (doesn't look like in the pics, but just being cautious) you should not drill it, and may need outside brackets to the boiler supports, or the boat frames, to hold the elbow.
Good luck with it. JD