One way is to make an artificial cave, such as plastic box or bowl, and cut out an entrance. Be sure the box is dark, not see through/opaque. Using a hot glue gun, glue a sponge on the top of it, and soak the sponge to maintain/increase humidity. Best to have few boxes set up, and you can dry them out, rotating them.
Monitor carefully and avoid yellow sponge, which my tortoises seemed attracted to and would probably eat(prevent that). Best if box is high enough the tortoise won't try to eat the sponge. EJ taught me this and it worked well. I also tried a pipe for a hide, with a damp sponge in the end and a barrier so the tortoise couldn't eat it, it worked OK but harder to clean than the box.
Others use coconut coire or orchid bark substrate, and just keep a corner damp, with a plastic hide box over the damp area. Give your tortoise choices, give them a hide box that is humid, another that is dryer, and see which it chooses. Choices are good, light/dark, warm/cool, damp/dry, rough/smooth, etc.
Understand humidity is not standing water and a swampy morass, it is humidity in the microclimate within the hide area. DO NOT allow your tortoise to sit in a wet area long term, but soaking in water bowl is fine as long as it can dry out.
How are other keepers providing humidity to hatchlings/juveniles to prevent pyramiding?
good luck, Vic