Jon check out the ash experiment thread I posted. One of the last cooks I did was a Low-n-slow. The 19" has a huge amount of space to hold ash so you won't have a problem down below. Because of the lump basket design you won't have issues with clogging in there either. A basket or grate to hold lump is a far superior setup (for not clogging) compared to a plate with holes.
I noticed Cassiopeia likes burn right to left. If I'm doing a Low-n-slow I'll start the fire on the very right side of the lump basket. Fill the lump basket all the way up to the handles and don't worry about it. I have yet to run out and I've done some really long multiple cook/temp sessions.
Charles - Prometheus 16.5", Cassiopeia 19" TT