The cold conduit is a concern. what i had in mind was to build a bottom board with open bottom and screen, put the slats under the screen. This way the bees would not be in direct contact with the metel pipe.
This defeats the purpose of a slatted rack. The advantage of the Slatted rack is that besides providing a thermal air layer at the bottom of the hive it also gives the excess bees a place to hang out so they aren't bearding and blocking the entrance.
This would let the air flow work and let the mites fall thru. Just looks like the normal rack would loose some of the efect of a open mesh bottom. The dead air space is a good thing. Losing mites is good. life is good.
Okay, but where do all those excess bees go on a hot day? Back to clogging up the entrance.