There is an American patented device called Mite Zapper or something which involves wires in the foundation being heated electrically.
The momma varroa is in the brood food snorknel up waiting, when the heat rises suddenly and kills it.
As far as I can tell reading between the lines the larva mostly gets killed as well (not good)

You may have used the type of hand warmer with the liquid which you click a disk or otherwise knock it and it then goes solid giving off heat for about 1hr.
The heat output is constant temperature around 50'C Max

My suggestion for truly organic control would be a thin version of the hand warmer in between two foundation sheets to form 1 complete comb.
At the time all the brood has been sealed trigger the reaction to give off the heat. Moma wont be at that end of the cell but junior the only male will be because he hangs out at the base of the cell on the 'Faecal Accumulation' waiting for the ladies (his sisters !!) to appear.

I wrote an article for Scottish Beekeeper some time ago including the formula for making your own DIY hand warmer contents.
Just a bit of fun really but in my own experiments varroa dropped onto wax cappings containing honey heated to just 40'C died almost instantly.

I also experimented with a sprayer and water at about 80'C which by the time it had produced a fine spray was detected at 40'C
Sprayed that on live varroa and it stunned them temporarily but they soon recovered