@Joe, don't worry about covering up the 3 sides for now... Don't put the heat pads under the tank; the metal mesh lid will help humidity escape, yes. You can cover half of the lid with a damp hand cloth or plexiglass - glass. Just leave a little bit of area for ventilation and a lamp if you're using one with the heat pads. To make your frog feel secure, since you're not doing the 3 sides covered, get some artificial plants or hides to let him hide under during the day.