A baby vice grips might be worth throwing in the bottom.
They make a purpose built clamp just for doing this. I will look
around and see if I have a few. IBM gave us 2 in a kit for replacing
the radiators in water cooled machines. I used to have a bunch but I
am not sure where I would start to look ;-)
The problem with vice grips is the teeth on the jaws is tough on the
hose.
This thing was round with a thumb screw to tighten it up.
I think the perfect solution would be a 3 way valve with a stub of
hose that you could poke into a jug to drain the tank. If I was going
to install a valve, that is what I would look for.
This shows the petcock the owner installed in the Honda. Pretty good idea.