Rain system in water dragon terrarium?

i have not gotten my water dragon yet, but im having trouble keeping the humidity in. Its a 36x17x17 screen terrarium, but i covered all the sides with, trash bags and clear plastic wrap and a wet tshirt on top, so its pretty sealed off. i have a large pool under the heat and 2 live plants, and 1 fake. but when i mist it goes to around 99% then lowers to 80% within 30 min. it goes down to like 45% within 1-1.5 hrs.. and i have a Tropic aire humidifier, but it really doesnt help. i need to know, which would be better. a rain system, or something else. please help me. i have the stuff to make a rain system but i dont know if that will be good for the water dragon. what would work to keep the humidity at around 80%?