Keep the substrate wet inside the box at all times, you could also put a small heat pad under the box that will evaporate the moisture into the air in the box raising the humidity. Keep in mind the air in the box may not always read the humidity level you want it to read, but if the substrate is constantly damp, you should be in good shape.
Here's a list of things to do to add humidity. You'll have to figure out how to apply them to your situation.
Cover as much of the top as possible.
Add a moisture friendly substrate, like orchid bark, coco coir, or cypress mulch.
Keep the substrate damp by dumping water on it, as needed.
Use a bigger shallow water bowl.
Mist the tort and the enclosure with plain water several times a day.
Humidify the whole room. You can use any baby humidifier sold at Walmart style stores.
Make a proper humid hide box, using a plastic shoe box with a lid.
Add live plants in and around the enclosure, if possible.
Add lots of live plants to the room.
Add lots more enclosures with damp substrate to the room. (This one's my favorite!)
thank you for all the ideas on humidity. I too have trouble keeping the humidity up. I decided to wet the substrate every day and I stir it with a big wooden spoon to get the underneath wet also and then I smooth it out again. It seems to work well but I think I willadd misting to the list also. Deanna