Ultra Cal or Hydrostone works great. I use Hydrostone to make plugs for vacuum form canopies. I paid $40 for 50 lbs.
I've made louver plugs too so I could copy a louver I had on a Yak cowl. (Don't use wood filler, it sucks up the moisture and cracks the mold).
I would assume for a larger mold you could use burlap and lay it up like a mummy??? I've never done it on a large model.
If anyone is local and wants to try it come and see me, I'll sell you 25 lbs of it