I believe that the laying media should be 50% sand and 50% organic compost. It needs to have no stones in it, and to be moist. It should also be at least half as deep as your sliders carapice is long, and as big as you can make it. sliders will sometimes dig many holes before choosing one to deposit their eggs in. Here's a link to a RES site where there are many people discussing gravid females and nesting.

Link