fine grade orchid bark or aspen shavings or coir and sand.
After thinking about it a bunch, I wouldn't use any substrate inside an outside hide. I'm just not sure that's necessary. An inside hide yes, but outside, I'm thinkin not so much. I personally don't use any in my outside pens, but with the exception of Bob, all my tortoises come in at night because I have 'coons and they eat torts. I know HermanniChris uses aspen shavings. You might just think about a bit of hay for him to cuddle in, glad you're going with the Hermanni, I don't think you will be disappointed. I'd also think about getting your Hermanni from Chris. Here's a link to his care sheet. I think that will help you too.
http://www.tortoiseforum.org/threads/hermanns-tortoise-care-sheet.63799/