I've heard of the Tupperware trick, but have never seen it applied to iguanas. It might be worth a try.
I forget how big your iguana's habitat is, but the only thing that helps at all is to use cool-mist humidifiers that you find at pharmacies. You put the humidifier(s) on the floor of the hab and aim the flow at the warmest zone.
Measuring the humidity is another thing. The cheap "reptile" gauges seem flimsy and I don't trust them. If I were to use them, I would set four in the hab, just to see the consistency between them, and count out one that is way off. I used to use more well constructed humidity gauges I found on the Internet.
Your iguana's body needs 80% relative humidity in one of the heat zones in the hab. The warm zones will always have less humidity than the cool zones. I used to strive for over 60% RH, wishing I could get it to 80%. It's not easy to humidify an iguanas hab, especially if it's not almost air tight. Mesh habs are hopeless, and the iguana will always live in RH that is comfortable to humans, which is around 30%- 50%, no matter how many humidifiers are in the mesh cage.