This year, I went without substrate. I bought one of those light diffusers and cut it to fit the tub, topped that with the plastic "yarn project" stuff (kinda stiff, with holes small enough that the hatchlings couldn't go through it), all raised off the bottom of the tub by 1 inch stainless tubing sections, which kept the eggs about an inch over the top of the water. Everything can be sanitized in the dishwasher. I'll try to get some pics posted of the final assembly. Only had 5 eggs, but all hatched without any problems.
Light diffuser:
http://granmoresuspendedceilings.com..._diffusers.jpg
Yarn plastic:
http://3.bp.blogspot.com/_SddhpD8Q7U...lastic+001.JPG
Stainless tubing:
http://www.boiler-tubes.com/pic/Stainless_Tubings1.jpg