I'm sure you have already found lots of other threads with great ideas on what to use. But just in case here are some materials I've seen used.
Foam and grout
Foam and Cement
those are the most popular materials used to make fake rocks and other props. I think a hide could be almost anything, but if you wanting a humid hide, then a plastic bowl with the other stuff applied around it would work. Have fun
youll be needing gecko safe white and blue paints, perhaps with a enamel texture? You can make grout hides and then make little groves with a knife or such.