I think that could work.I would treat it like vermiculite (wet it enough that it sticks together, but no water drips when squeezed), and pack it lightly around the egg, then plastic tub over top. I'm assuming from your previous post that the egg is stuck to the tank bottom, but if not, set up an egg box and just move the egg into there (keep in tank if that's the only way to keep it warm enough).