you need to make a mold of the inside of the part.for vacum forming i have used plaster of paris but have found that though it feel smoothes when the hot plastic covers the plasrer small pockmark develop from heated air in the plaster,i have heard that clear coating with poly u will solve that problem but have nut tried it.another substance to make a muld/plug for your purpose would be bondo ,but you need a negative type mold for best results/if you use a positive mold i would suggest using the petit hoppypoxy easy does it method,where you fit a ballon over the finished layup to get a smooth finish when the epoxy cures.