Buggerdtp,

Your substrate can help with the humidity as well. PDF don't require ventilation (the air circulation from opening the enclosure to feed or mist the terrarium will be enough) so your top should be all glass. This will greatly help to keep the humidity up. Also if you include some moss in your substrate. I like to use the ABG mix for the middle layer of my substrate and then I top that with about 1 inch of Sphagnum Moss. The moss holds in quite a bit of water and will help keep the humidity up. It will also help keep the frogs from having ABG mix stuck to them. I then cover the moss in a layer of Leaf liter. I prefer Magnolia leaves, but that is just personal preference.

If you do use Sphagnum moss, when you are putting it in the tank soak it in dechlorinated water and the ring it out so it is damp but not dripping wet.

The misting + the Damp moss will keep the humidity quite high. The false bottom is simply to collect execs water and keep it from swamping out the substrate. If you would like the water to help with humidity you could look at adding some Leca or Hydraballs (expanded clay) to the false bottom area. The Leca will absorb some water and then when you drain the False bottom will begin to dry out and release humidity.

Hope this helps some.