What brand and type of humidifier is this? Does it have its own blower? Also the return air does not have a plenum. The plenum on an updraft furnace is above the furnace and where the air conditioner coil sits in. You do not want to cut into the plenum where the a/c coil is but up above the coil. When you say return cold air plenum and hot air plenum has me wondering cause with a furnace/a/c combo, the plenum is the same and will be hot with the furnace and cold with the a/c.
The brands I am acquainted with such as general and aprilaire are ones that you would want to mount above the plenum but still include a bypass over to the return air. You need this bypass to get the air flowing through it right.