If not using a natural substrate you may consider it? Something like a basic 60/40 sand and topsoil mix? It can hold a reserve of water that would evaporate and raise humidity. Another idea is to take a water soaked paper towel and put it in a corner. As it dries it will raise humidity. Also simply misting down the enclosure in the morning. Like a morning dew.. Not a total monsoon. Can help too.
I'm slowly switching all my enclosures to these to measure humidity and temp. They are very accurrate and easy to use with your smartphone. It will also record the temp and humidity to look back on. If you slurge for two you can put one one each side, but if one just put in center of enclosure to get the average temp and humidity. I just stick them to the back wall with a command strip.