I had exactly the same problem as you. Humidity at 35%

I bought a piece of thin lexan and had it cut to fit over the top of the tank, leaving about 2" on one end uncovered to allow air exchange.

My humidity is now at a steady 50% +/-5% and I would imagine you could adjust that to be more or less by how much screen you leave uncovered. Just make sure to leave enough open to allow good air exchange.