Whole House humidifier installation

I have been thinkling of buying one at lowes. I have propne heat in the crawl space. The ductwork coming out of the unit is the "soft stuff, a large round one deeging the small lines to the rest of the house.
How can I install this in such a system? Will I have to get metal ductwork?
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You can...and you cant. Anything can be done, but the question is, who is the dumbass that installed that system with flex, and not at least a plenum for the return and supply side?
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