Great ingenuity!
As far as the ash eating away at your drum....I THINK that it's only corrosive when it gets wet. Could be wrong, but I'm pretty sure.
Either way, if you're not into the "trash can dump" manouver, then doing it that way is necessary! Good stuff!
PLUS....great thinking making it rounded in shape to get into the corners of that round drum!
Whatever you do.....I hope you're not throwing away that charcoal!!!!