The best substrate is a mixture of fertilizer-free topsoil, peat moss, compost with soft play sand or ReptiBark.
Over the years I have found that making your own mix is the best way to achieve a stable yet porous (well-draining) substrate that works perfectly for the unique habitat setup you choose for your redfoot.
For example, the type of substrate I chose for Malti’s indoor enclosure when she was a hatchling and very young juvenile was weighted heavily towards ReptiBark with sphagnum moss.
Now that Malti is an adult and lives outdoors for much of the year in a far larger enclosure, I use a combination of fertilizer-free organic plain topsoil mixed with other elements such as hay, moss, ReptiBark and leaf litter. This mixture keeps the soil from turning into thick mud and blocking the enclosure’s drainage holes (scary!!) during the torrential rains we sometimes get. But it also provides enough soil content so Malti can dig to create the mud wallows and cozy hides she so loves.
Ultimately, choosing the right substrate will help in every way to achieve the other goals you want for your redfoot’s enclosure. A good substrate will lock in humidity and heat while still allowing excess moisture to drain so conditions don’t get too soggy. It will also help your redfoot enjoy the enrichment and safety of burrowing and digging.
For hatchlings and juvenile redfoots in particular, adding in plentiful amounts of moist (not sopping) sphaghnum moss offers “humidity caves” for hiding and easy rehydration.
You can take an empty, clean plastic tub, turn it over, cut an entry hole and pack it with sphagnum moss to make a simple humidity cave for your redfoot to enter and exit for some extra beneficial humidity. There are also pre-made reptile hides and log hide-outs that may work if you prefer.