What's wrong with the tried and true, shovel the coals into a pan and make a made dash for the ash barrel as flames flare up at your nose!
I've done a pretty good job of learning to judge my charcoal use. I guage my refuelling by the time left to cook and most of my coals are burnt by the end of the cook time. The few coals left, I shovel out into a bucket. I like the water in a pan idea Theron!