I used some sort of mulch, probably fir or cypress, and then hide boxes with moistened spagnum moss. The key for me seemed to be finding the right humidity and moisture levels so they would not soak constantly.

Neat snakes and I miss working with them.
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Current snakes:

0.0.1 Gonyosoma oxycephala - Java locale (green)

0.2 Gonyosoma oxycephala - Malaysian locale (green)

1.2 Gonyosoma oxycephala - Java local (green)

2.2 Gonyosoma janseni - Seleyar locale (all black)

1.2 Gonyosoma janseni - Celebes locale (Black & Tan)