If you have room in your tank pick up some kind of hollow ornament like this skull I have and pack it tight full with moss. You'll need to get it damp to pack it in really tight. Then you can just pour water in it and get it nice and damp and set it in your enclosure. Also go to a hardware store or Lowe's for example and get some aluminum tape. Tape off your screen top leaving only an area for your heat lamp.
The ultimate solution to humidity issues is a PVC enclosure like my T8. I can keep my humidity easily around 65% just by keeping the water bowl full. I only dampen the skull to bump it up when he's in shed.