Please help us with Royal Walnut Moth larva

[Boggs]

My son and I found a larva and identified it as a Hickory Horned Devil.
The larva is identical to the picture in our Peterson First Guide to
Caterpillars.  The book states that the pupa overwinters in the soil.

Can someone give us some guidance on how we can accomplish this in a
cage.  We have enjoyed watching lots of caterpillars form cocoons and
chrysalises, but have never and any experience with one that forms in
the soil.

So far we have put him in an aquarium with about 5 inches of dirt,
mostly red clay (North Carolina).  We are interested to know if it
matters what kind of dirt, how much, and if we should keep him inside or
outside.

Any comments, instruction, or information about where we might find
answers is appreciated.

Jim and Scott