You definitely want to use a down-draft system if you have the option, it's much less likely to bounce the cones around when they get close to dry.
I finally got around to revising my totally rudimentary system into something that works stunningly well. I put almost six pounds of Centennial in there late Saturday afternoon (could easily handle twice that) and they're nearly done already, with the fans at their lowest speed the whole way. I'm sure it could scale vertically by at least a factor of two before air resistance might matter.
I stuck a post about it here. Some serendipity involved, the house screens were big enough to use a pair of 20" box fans...