The Ontario Beekeepers Association worked with University of Guelph 
on an approach using Homsote board in perforated Ziplock veggie bags,
and they may have some tips that may be of value.  There was a long 
and very detailed discussion about this on the UseNet group 
sci.agriculture.beekeeping, which can be searched with 
"Google Groups".

The main issue is temperature versus "dose".  For any specific
pair of day/night temperatures, there is a does that will kill
the most mites without killing bees.  Problem is, no one has
bothered to plot enough points to draw the line on the graph
that would be useful for a homebrew formic user.

I'd check out Marion Ellis and his work with Oxalic acid
at U Nebraska.  Easier, safer for employees, and cheaper
on a per-hive cost basis than formic.
 

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