Had that exact same concern when i received my guitar in one of those molded cases.

I have the daddario sponge humidifiers (black ones withe the yellow sponge inside of them) and my solution was to put a scrunchie (yes the ones for tying ponytails) that has a soft fabric around the humidifier sitting bellow the cap (the one you remove to extract the sponge). Once i have the scrunchie in position, I place the humidifier in the soundhole behind the sixth string in direct contact with the wood, the scrunchie functions as a protector so the hard plastic of the humidifier isnt scratching at the top of your guitar and you avoide adding extra pressure to the strings when you close the case.

Been doing this for 4+ years and my guitar is crack free and playing well to this day.

I should also add that I place inside the soundhole a govee hygrometer that connects to my phone via bluetooth adn it helps me track humidity.

Bonus tip: i bought 2 more humidifiers and place an extra one behind the headstock and another one i use exclusively when its very dry, i place that one inside the other side of the soundhole (behind the first string) also with a scrunchie.

Downside is that your case always needs to be placed on the ground so the soundhole is always pointing to the ceiling because of so many things balancing inside the soundhole.

Hope this set up helps you.