"Just Gorilla-tape a garden hose to the heater core!"
I do not have any idea how to seal Gorilla tape around a hose and a smaller and oddly-shaped connection. I tried twice and failed twice, although the water leaked out of the tape, not the core.
So, I cut a piece of inner tube, which barely fit over the end of the hose. I used a hose clamp and then a zip tie, as before, over the heater core opening.
The tube burst, but the core seemed to hold. I am pretty sure that was the AutoZone one.