Is this for the Chewie underskull? If so, I would not recommend a box mold for that piece (it's way too big for that and will result in the mold costing much more). I would go with a brush-on mold or a silicone matrix mold (harder to do in my opinion, but they use very little molding material - just finished doing one for a tusken underskull).
Smooth-on makes both a brush-on urethane and a brush-on silicone. The urethane is less expensive, but you need to make sure that you adequately release the mold or the cast will get stuck in the mold (urethane is an adhesive). Silicone molds don't require a release agent (as long as you aren't casting more silicone in them), but the mold material is more expensive.
In either case, if you do a brush-on mold, you will need to make a 2 piece rigid mother mold after your mold material cures to insure the mold keeps its shape (I can send you links to tutorials on making mother molds if you need them).
Randy
