Getting a plug (core) out without damage can be difficult. Use as much draft as possible (tapered sides). If scale shape doesn't allow health draft, consider making the core from two or three pieces. A sandwich with a wedge shaped center sometimes works well; the wedge only contacts resin on the narrow sides; you pull it out and then pull the outter pieces towards the middle and remove (collapsing core). However you do it, make sure you have a good handle on the plug (like threaded rod) and some way to grab the molded part (like a molded flange). be sure to extend the fiber glass/plug far enough so you have material to trim/clean up damage etc. I don't know how big the piece is but balsa is expensive. Higher density foam is more durable (like urethane "Balsafoam"). Another option is (machinists) wax. It machines/carves nicely. Machinists use it to test CNC programs before committing metal. Good luck - post a picture!!