Tupperware containing water is there to keep RH high like you say. If you can fit a tall glass instead and have a big enough mono to fit a second bag that should work.

Without some kind of independent water source to keep RH high the fruits tend to dry too much as they leave the bag and if you spray them to recover they can get damaged (I've been there). I've also tried simply pouring water into the bottom of the monotub, but spores got in the water and the situation did not look as clean and tidy as when the water is kept separate and to the side.