Usually the rule of 3rds is a safe bet. They aren't fish, so they will need rest on land, and they like to walk around too!
I'll show you my favorite habitat--this was actually meant for firebelly toads, but their care is pretty similar and the habitat works for either
He used hot glue to glue together the island! He held the island up with aquarium driftwood.. Oh, that lump on the left is where he hid the filter! XD the piece is removable, but looks cool!
The reason I mention this is I think it would be a fun craft! I actually would like to make something like this someday :D