FWIW - I've come to the conclusion that Varroa is going to remain with us for a very long time indeed, and so it seems appropriate that the basic beehive design be modified to take account of this. So - all my hives are now to be fitted with 'cavity bases' to accommodate a band-heater equivalent of the Varrox. [a simple steel can fitted with a 150W band-heater; cost around 3GBP ($4 US) each; up to 8 hives dosed simultaneously, controlled by a timer/SSR - no need for accurate temperature control]
As the cavity will be around 3" high and a permanent feature, I'll be fitting removable Slatted Racks (CC Miller style) in each cavity to prevent wild comb being drawn there. Entrances will be above the Slatted Rack, not below them as currently.
Dosing 8 hives at a time, I'll probably allow 15 mins for each application, as speed will no longer be an issue. No need for personal protection either, as I'll be indoors during the treatment enjoying a coffee (or beer !).