Quotes:
"These tortoises need both an outdoor and indoor enclosure where ideally they can have free access to both areas in the summer. The indoor enclosure can be in a shed providing at least 180x120cm space for one tortoise; lined with a suitable substrate, such as a mixed loam compost base mixed with varying amounts of soft (play) sand. This needs to be of sufficient depth, about 50mm or 2 inches as a minimum. Increase the proportion of sand for a more arid habitat. Gravel, wood shavings, alfalfa pellets, calci-sand and cat litter are all unsuitable substrates"
and
"Large rocks for hiding, basking on and climbing over, and an area of deep insulating material (shredded paper or leaves) in which the tortoise can bury itself in, are other essentials"
Source:
https://www.rspca.org.uk/ImageLocator/LocateAsset?asset=document&assetId=1232716693105&mode=prd
So a couple of inches of a loam/sand mix underneath lots and lots of shredded paper, perhaps?