A tank will be fine, but it does take more work First, why is your hot side at 97? That is a little too high, which makes me think your UTH is uncontrolled. Do you have a thermostat or rheostat plugged into your under tank heater(UTH)?

I don't think you should put the UTH in the middle to help 'spread' the heat because, well it won't. It may a little with some foil on the under side, but it may also make your bp feel more exposed, having a hide right in the middle. My suggestion would be to grab another UTH (you will need a control for both) and put it on the cool side set to 85. You can also use a lamp to help keep the cooler side warm, but lamps do lower humidity.