I had a 10" PVC section cut in half lengthwise for a cave, then I filled the opening with moistened sphagnum moss. A couple three times a week I had to take it out and re-moisten it. And the baby never went in there on his own. I had to put him in every night. You can see the PVB here, but there's no moss inside yet:
A storage container upside down with a door cut in it works well. Sorry, I don't have pictures. I know Tom has posted pics though. If you search humid hide, you will find a bunch of ideas.
I use black dishwashing tubs from Walmart. They only cost $1.82. Flip them over and cut out a door hole just big enough for the tortoise. Enlarge the hole as your tortoise grows. Push the edges down into the substrate and keep the area of the substrate under the humid hide, more damp than the rest of the enclosure. Expect to have to re-wet the substrate periodically depending on how dry your area is.
You can see pics here in this thread: http://www.tortoiseforum.org/threads/russian-tortoise-care-sheet.80698/
Its a Russian tortoise care sheet, but I use the same humid hides for all my species. Just don't follow the rest of that care sheet for a sulcata. Posting it here for the pics only.