I would think you could use wigglers as filler, though it might be easier to use a hot water bottle. The nice thing about filling the empty space in your bator is that the radiant heat can come in real handy in a power outage and you could fill the hot water bottle with hot water repeatedly if you have a long outage, as long as you have alternative way to heat the water...
Edited to ad: See how quick PC is to answer questions, lol.