I think sponge material might be a useful substrate for some aquatic situations, but not for fully terrestrial animals. The only substrate that makes sense for efts (in my mind, which may be limited!) is soil or soil-like substances. I've tried various alternatives (wet gravel, damp paper towels, etc), but I've gradually reached the conclusion that good old dirt is actually CLEANER (and less hassle) than any other substrate. I know this sounds crazy, but dirt has miraculous properties of breaking down wastes. When the animals are sitting on dirt, they should be able to absorb the moisture they need thru their skin. Noto efts are absolutely hydrophobic - there is no way to keep them in/on/with water.
My failures with N. v. dorsalis, which I kept for 20+ years but never raised successfully:
Year 2000-2001. Raised larvae OK. Kept the efts in a terrarium with a steady diet of fruit flies for about a year. They did not appear to grow much, if at all. Then one by one they began to die. I tried various things to keep the remaining ones alive - I even got a couple of them to go aquatic and eat blackworms. One of the little females even laid a few infertile eggs. But they all died within a span of a few months.
Year 2002. Raised larvae OK. When they reached metamorphosis they changed color, but kept nubs of their gills and stayed in the water. Odd. Then, one by one, they all died. I have no idea why they didn't morph fully, or why they died. They looked and acted perfectly healthy.
(sorry, I couldn't resist)