What's a good block filler?

I'm working on a project, and I need to fill about 1 quart's worth cavity. It's a magnesium alloy casing. I expect temperatures to stay below 200* F. The thickest portion will be about 1.25" - 1.5". It will be exposed to transmission type fluid.

I need to know of an inexpensive filler that I could use. I may be making several of these in the near future, and would'nt mind buying in bulk once I've found something that works well. I'd rather not have to bake cure this specific part as it is magnesium, and could ignite as a result of some freak accident.

Are there any suggestions as to what to use or not use?

Thanks in advance.