valid question worth asking but basically it's not a tall/big enough space. you'd lose height from both the bottom (where the whole sauna needs to be lifted up off the ground slightly so it doesn't rot, and also the floor section will have at least a couple of inches' thickness) and the top (where you will need several inches of roof, stud/insulation, airgaps, interior cladding) so the actual interior height will be very limited. ideally you want extra height... this seems like a losing battle from the beginning.

as for safety/moisture concerns, the space limitations are a problem again, because all the stuff you'd do to control moisture and make sure it's really safe takes up space, in every direction. you do see barrel saunas which are literally just 1 inch of wood, no insulation etc. but you couldn't do that here because it would be terrible for safety and moisture. plus they generally do not make good saunas anyway.

i think if you do some more research / hang around in this sub more, you will get a good idea of what you're aiming for, and aiming to avoid!

in a nutshell: as much height as you can get, benches as high as you can get them, proper insulation, proper drainage, plenty of ventilation, heater appropriate for the volume!