There is a material called Coremat. Essentially you put a few layers of glass then the Coremat goes down and saturated with resin, then another few layers of glass. You can bulk a mold to 3/8" pretty easily with this stuff.

The other option is tooling compounds. These are highly filled resin systems that can be handled like dough. The doughy parts are mixed together and then packed on the first glass layers to varying thickness. Then more glass.

Another option is Poraver. This stuff is like 2-4mm hollow microspheres. You mix about 50 grams of resin to 1 liter of Poraver. This gets packed onto the mold or packed within dams. It has a higher compression strength than the bean bag filler but is slightly heavier, more expensive, and harder to acquire. Again, it should be used between layers of glass.

The above techniques allows the mold to be constructed in one shot if needed.