I'm bumping this up because I want to keep celebrating this effective, cheap mod. And also because I wanted to share some field data.
Now the lid thermometer in my SSP isn't especially accurate, but here's what I've found so far:
1 chimney of KBB = at least an hour right at 400 degrees
1 chimney of Kingsford Comp. = at least an hour right at 450.
The fire brick fence really gives a true indirect heat, and I've been thinking about some kind of 'catch' or 'keeper', because they are prone to falling down once I've got everything set up and I give the grates a quick scrape. So far some wire bent into a U shape makes a decent clip, but I'll keep working on something better.