why do you think you need to? the sublimate rises, desublimates then drifts over the bees, happens in seconds it doesn't really matter whether the OMF is open as when it hits the floor, open or not, that stuff is redundant.
You could always make the eke I use for varrox vaping through the OMF on nucs and just sit the nuc on top to close it up during vaping - a simple solution but it will work for those nucs with 'feet.
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I think a lot of this hermetically sealing up and closing every little crack in the hive is just vestigial thinking from the days of apiguard and the likes when everyone accepted that the 'vapour' that lurked in the hives throughout treatment was the magic ingredient