Sand and peat is a good way to go.Mix them together and then put about 1/2" or so of just sand over the top of that so when they dig a nest and bury it you'll be able to find it easier.

Also, spotteds like to lay their eggs at the base of a plant or a piece of drift wood.1 of my females lays her eggs backed up to a piece of wood every time...Now if I could just get some fertile eggs....Post some pics of your enclosure if ya have any, we love pics around here